My answer is in the light of Fiqah Hanfi........
Both of your male and female cousin can't marry with each other or with any of each other's brother n sister.....
your male cousin's brothers and sisters can marry to your female cousin's brothers n sisters.
I don't know why your parents didn't care about this before your cousins got that far.....
well although u need to be consulted by some Molana..but as for as my knowledge is concerned,they cant..and its nothing like that,if they do this thing then its possible.i dont think its possible in any condition!!
This is pretty serious issue. In any circumstances they cant get married if the girl was breast fed by the boys mother while the girl was under the age of two calendar years. It doesn't matter if she was breast fed once or more times - onlyl once one full mouth of milk swolled by the child is more than enough for this issue. The only thing matters here is age, if child is over two years of age then it doesn't matter but if under that age then it matters the most.
She is like a real sister to the boy & if they both marry each other it will be brother marrying his own sister. They must not involve with each other. Anyone telling you that they can get married is complete stupid. There are enough references about this issue in Quran - search it for yourself.
By the way Prophet was breat fed by some stranger & he always treated the the children of that woman has his own siblings. In Urdu this term is called "Razaee Behan/Bhaee".