Pirkei D’Rabbi Eliezer says that they married their sisters specifically to avoid having to marry women from other nations:
????? ??????? ?? ???? ?? ??? ???? ??? ???? ?? ???? ???? ???????? ???? ???? ??? ??? ?????? ???? ?????? ????? ????? ??? ??? ???? ??? ???
As to how they could marry their sisters if they kept the Torah, the Daas Zekeinim asks this (which I guess is a source benignuman, that they kept the Torah) and answers ??”? ?? ???? ??? ????? ?? ????? ??”? ?????? ????”? ?? ???? ????? ??? ??????? ??? ??? ??? ????? ??? ?????? i.e. they kept whichever parts they wanted. Interestingly, the Daas Zekeinim concludes that based on the Gemara (Pesachim 119b) which says that Yaakov will decline to do the ??? ?? ???? because he married two sisters, apparently there is ??? ???? since in the future there will be a Torah, and Yaakov knowingly married them anyway because it was the only way he could marry Tidkanios that would produce all 12 of his sons.