Meltdown or tantrum?

Ok, so the basic simplistic rundown of the difference is;

A tantrum= A desperate attempt to get something the child wants.
Child, to the extent that children do at the age they are, knows what they are doing.
Child can be distracted or placated, giving the child what they wanted will stop the tantrum in its tracks.
Child recovers fairly quickly.
All children have these, the expectation is that as they grow up they find better ways of getting what they want/coping with not getting it. An autistic child may get this more slowly (as the ability to cope with one's own emotions is something that is directly negatively affected in autism), which should be allowed for, but they should also be learning, like NT children, that tantrums are not the 'right' way to deal with things.

A meltdown= An explosion of unprocessed negative emotion, often but not always built up over a long time (the course of a day, or even several).
Child is almost entirely on autopilot and doesn't know why they are doing what they are doing. They may even be alarmed by the intensity of it after the fact, which is a dead giveaway.
Child cannot be reasoned with, distracted, placated or calmed by giving them what they appear to want (the apparent trigger is rarely the only trigger, more like the "final straw").
Child is extremely exhausted/emotionally drained afterwards. 
Very much an autism thing, so it would be both pointless and unfair to expect a child not to have them or discipline for them (at any age; adults still have meltdowns, but these tend to be less frequent due to learning better coping mechanisms for the actual causes over time).


"having to change games etc to accommodate her sisters wants"
This line jumped out at me, because changing activity is a classic example of something autistic people experience very differently and much more negatively than neurotypical (non-autistic) people seem to. 

Being asked to suddenly change activity, to an autistic person, can evoke a visceral feeling of discomfort and repulsion. There is a compulsive need to finish the task that was being focused on before the interruption and not being able to act on it can feel unbearably uncomfortable and provoke an immediate and extreme reaction, especially as a child (again, purely because autistic adults have generally developed coping mechanisms with time and experience). I have likened it to being asked to stop using the toilet in the middle, which is not a pleasant analogy (so apologies for that mental image) but one that I feel is the most accurate! 

Even before I was diagnosed (I wasn't even flagged up as "possibly autistic" until I was 14, because girls generally weren't when I was a child), my mum used to use strategies such as giving "X amount of time until we [change activity]", without the expectation of a verbal response, then reminding again at intervals over the course of an hour or more to gradually let me cool down from the thing I was focused on. She called it "planting a seed", which is a metaphor I rather like.
I also like the analogy of landing a plane; if you just plunge the nose down when you're directly over the airport it all ends in fiery doom (meltdown). You need to extend the landing gear and the flaps and raise the nose and gradually slow down before you even think about touching down on that runway. 

"not getting her own way"
What do you mean, specifically, by "not getting her own way"? 
If this relates to "things she is going to do" or "things that are going to happen", there may be an issue of routine/script deviation behind this, which is another classic meltdown trigger.

Autistic children, on the whole, need routine and knowledge of what is going to happen to them in the immediate future in order to feel secure.
If you've been doing some reading around as part of the diagnostic process you have probably already read that a change in an expected routine can be very distressing for an autistic child. 
One of the big problems that most people don't foresee, however, comes when there isn't an expected schedule (provided from an outside source, e.g. a parent) for a particular activity or time of day. The child can then sometimes make up their own "script" as a self-reassuring thing under these circumstances. "I'm going to play this until I achieve X and then I'm going to have a drink and then I'm going to..." and not tell anyone else about it.
When a parent or other person then tries to get the child to do something else, they then unknowingly hit the "routine change" trigger and chaos ensues!

Other meltdown triggers that build up throughout the day include; Sensory input (textures, too-bright lights, noises that are too loud or at a pitch the person is very sensitive too) that a neurotypical person might not even notice but cause the autistic person a significant amount of stress, Social situations (autistic people often find these take a lot of mental energy to navigate), Perceived injustices/upset to themselves or others; contrary to the common belief that autistic people lack empathy, that applies to cognitive empathy (which is being able to "put oneself in another's shoes" and accurately recognise another's emotional state) only. Affective empathy is usually normal and some of us even have an unusually strong response when we do realise another's suffering exists (and react to it with extreme personal distress). 

Discipline in itself is something you may want to approach in a different way than you would do with a neurotypical child, because confrontation is something many autistic people find deeply uncomfortable/hard to process and could therefore contribute to a meltdown in itself. Autistic kids tend to respond much better to reward systems (additive for wanted behaviour only, as we may react very strongly if we feel 'points' have been taken away unfairly; which they might be if it's for a behaviour we can't help. Best not to invite such scenarios by subtracting at all).

The only way to stop meltdowns is prevention, be that reducing the exposure to triggers (ideal) or teaching your daughter coping strategies such as taking herself off somewhere quiet or wrapping herself in a blanket (deep pressure can be soothing) when she feels a meltdown is imminent. This may or may not be possible because the ability to identify one's own emotions is often reduced with autism. It depends on your daughter, but I would absolutely not expect it to be something she can do reliably at seven.

My family found this book (available as a free PDF) really helpful when I was first diagnosed. 

http://autismforthvalley.co.uk/files/5314/4595/7798/Attwood-Tony-The-Complete-Guide-to-Aspergers-Syndrome.pdf

How old is your other daughter? Is she old enough to start learning that your probably-autistic daughter has a different way of thinking and taking that into account in her interactions with her sister? Sharing that information, teaching each child to intellectually understand the other's thought processes in equal measure, might significantly help their relationship going forward and nip any bad feelings that might result from treating them differently (which is really a necessity, because they are different) in the bud.
I used to have an extremely antagonistic relationship with my sibling, before I was diagnosed and learned my coping strategies and he started to talk to me about/read about autism and learn the reasons behind the outward behaviour.


Hope some of this helps (and sorry it's such an essay; I've tried to break it into digestible chunks) 

Emma x

Wow Emma your awesome,,, You not only help educate parents you educate us all, I knew the differences but reading the many variables that can cause issues and melt downs is most enlightening, big thank you for being such a positive ambassador for autism and us. X()x

Wow Emma your awesome,,, You not only help educate parents you educate us all, I knew the differences but reading the many variables that can cause issues and melt downs is most enlightening, big thank you for being such a positive ambassador for autism and us. X()x

Thanks Lonewarrior!  That's a really lovely thing to hear and I'm glad it was useful for you too x