Please encourage her to be careful, there is a cycle to abuse and it includes the "I'm sorry" and flowers (this is called the honeymoon phase). It never last and each attack is usually worse than the last.
AMEN to that. It also often includes an incredibly manipulative manner in the abuser that twists the facts and leaves the abused woman wondering if the abuse was somehow her fault or if she did something wrong... he can even convince her that she deserved it.
The bottom line is this: even if I DO say something inappropriate or "wrong", it still doesn't give anyone an excuse to abuse me mentally, physically or emotionally. Abuse is wrong. PERIOD.
I am so very sorry your sister had to go thru that. Please have her get in touch with a shelter so they can help her and the baby.
Hugs
Cats