Its up to you who you got out with not your cousin. Its simple your cousin will get used to it a start acting like a normal human after a while…
This happend to me but i told my brother to shut up and get used to it 
If your happy, then your family should be happy for you too. Its not like this new girl is an axe welding maniac? Is she ;)? If not, then they’ve no grounds to be fussy/upset/grumpy if you like the girl
