Salaam Alaykum ;
Quran forbids marriage between children who were breastfeeded by same mother. A sis wrote about this problem and many of us gave detailed ans there . So , pl. try to find that thread.
if ur aunt does not remember how many times , Don't take any risk . Don't marry any of her daughters as they could be ur sisters .
Here is the verse :
Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your fathers sisters, your mothers sisters, your brothers daughters, your sisters daughters, your
foster mother who gave you suck,
your foster milk suckling sisters, your wives mothers, your step daughters under your guardianship, born of your wives to whom you have gone in - but there is no sin on you if you have not gone in them (to marry their daughters), - the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allah is OftForgiving, Most Merciful.
( سورة النساء , An-Nisa, Chapter
#4, Verse
#23)