Dear L.,
it sounds that you have very nice and warm heart.
it seems to me as well you are in good relationship with your EX and his parents .
first talk to your EX and find out when is the best time for you to go there while his sisters are not there . without saying anything about your feelings just make it as you do not want to make his dad upset if his sister ended up saying something bad to you .
if i am in your shoes i would do that .
i used to have a boyfriend for 5 years and i was in good relation with his family , and still until today after 12 years of being married to another guy i still call his parents and still go and visit them whenever i am in the area .
they know that when his wife over there i do not like to go so she would not start any problems .
so keep the peace for the sake of your kids and take them with you after you are sure that there is no sisters over there going to make any problems for you.
good luck for you :)