Your sister-in-law has burned that bridge. She cannot be trusted. You should speak to her and let her know how selfish and deceitful her behavior was regarding her maternity leave and returning to work. While people do those things sometimes in order to get as much money out of their jobs as possible, she is family. She should not have taken advantage of you in that way. Unless she is your only option, don't hire her back.
"I rise today with my newborn Sam, who's just four weeks old. Unfortunately, I wasn't given the opportunity to vote remotely after giving birth, but I wasn't going to let that stop me from being here to represent my constituents and vote no on this disastrous Republican budget proposal."