What to do about shitty family members

I have a son who is 3 and currently we are trying to figure out why he has sleep apnea and how to correct it he’s been to numerous doctors and many sleep studies and they are not sure what’s going on. My brother and his girlfriend (exwife, don’t ask) seems to always put their two cents in where it doesn’t belong when it comes to my child. They always bring up “well maybe it’s not sleep apnea maybe he has autism”

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My child has been tested 3 times for autism over the year for speech Therepy and they have told me countless times he is not autistic. I’ve explained this to my brother and her countless times but they don’t seem to want to let it go. It constantly bugs me that they are trying to slap a label on my kid when that’s not even his issue. Even his doctors agree he has sleep apnea as they have ran multiple tests to prove it. How the hell can I get them to stop trying to label my son with autism when it’s really sleep apnea? Every time they say it it makes me want to scream at them. There’s a lot of issues within my family and this is just one of them that’s been bugging me a lot lately. Like if my son was autistic it would be different and I wouldn’t love him any less but I feel like they want to label him as autistic cause then they would have a reason to say he’s a problem child. I don’t get it and don’t know how to make it stop