I love the names you give to relatives, it's got a very high degree of precision. and i love this sentence on top of all.
I wrote my uncle is the brother of my mother and it does not accept it
Well, that is because 'bror' is not being used as a definite word in this clause... I think that what you translated would be something like 'Min moster är brodern av min mor' (as far as I could tell, subjected to corrections)... And besides I don't know if that is idiomatic or makes sense in Swedish... the form being asked to translate has a direct translation in english '... min mors bror' as 'my mother's brother'... That's why I think it is the only accepted translation in this sentence.