Husband cheated with sister

Hannah
I just found out tonight, at 7 months pregnant, that my husband and sister have been messing around since before we got married 5 years ago. Both say it stopped while she was dating someone else for a couple years, but still occasionally still have inappropriate conversations and flirtation. I don't even know what to do right now...
UPDATE:
Thanks for all the support from everyone! I appreciate all of the words encouragement and understanding. At this point, I am willing to try to see if we can move forward. Counseling and really focusing on fixing the problems to led us to get to this point. Not only am I 7 months pregnant, we also have a 2 1/2 year old. I need to at least try. And he's doing anything and everything he can to get me to stay. Once again, thank you guys!