GP's not helping with diagnosis - advice please

Thank you for all the help on here so far. Thank you for your comment Hope. She is still very young that is why the doctors refuse to acknowledge she might need help in this area and blaming my sisters parenting. My sister is a great mother who tries her best but would obviously benefit from support to help my niece. She has looked into everything, gone to hospital appointments, attended school meetings (when she was blamed forthr behaviour) she has in the last few months kept a log of everything. To answer your questions: She is obsessed with food, an unhealthy obsession for a 5 year old. She is not overweight (in fact she could probably do with putting more weight on) we have tried increasing her meal sizes and snacks (healthy and unhealthy) but once she's eaten she asks what she's having at the next meal. She talks about food, plays with pretend food, draws food. She experiences times of frustration (not tantrums) when something doesn't happen like she wants. Then when you ask her what's wrong, she can't express herself. E.g. walking in short grass with the dog (which she wanted to do) she starts crying and says the grass is too long. She has a main friend at school but doesn't really interact with any other children that she doesn't know or hasn't played with before. She's happy to play on her own. She doesn't appear distressed by symptoms but frustrated. She is very bright with reading, drawing, writing but cannot ride a bike\scooter (we have tried numerous times over years) she even struggles with running. Sorry for the length of this, I appreciate everyone's help. As I write this on here and answer the questions it just seems to make sense with her behaviour over the last few years.

Thank you for all the help on here so far. Thank you for your comment Hope. She is still very young that is why the doctors refuse to acknowledge she might need help in this area and blaming my sisters parenting. My sister is a great mother who tries her best but would obviously benefit from support to help my niece. She has looked into everything, gone to hospital appointments, attended school meetings (when she was blamed forthr behaviour) she has in the last few months kept a log of everything. To answer your questions: She is obsessed with food, an unhealthy obsession for a 5 year old. She is not overweight (in fact she could probably do with putting more weight on) we have tried increasing her meal sizes and snacks (healthy and unhealthy) but once she's eaten she asks what she's having at the next meal. She talks about food, plays with pretend food, draws food. She experiences times of frustration (not tantrums) when something doesn't happen like she wants. Then when you ask her what's wrong, she can't express herself. E.g. walking in short grass with the dog (which she wanted to do) she starts crying and says the grass is too long. She has a main friend at school but doesn't really interact with any other children that she doesn't know or hasn't played with before. She's happy to play on her own. She doesn't appear distressed by symptoms but frustrated. She is very bright with reading, drawing, writing but cannot ride a bike\scooter (we have tried numerous times over years) she even struggles with running. Sorry for the length of this, I appreciate everyone's help. As I write this on here and answer the questions it just seems to make sense with her behaviour over the last few years.