<human>: My sister and her boyfriend have been together for nearly 2 years now. She is about 12 weeks pregnant.

Today, my sister was venting to me about her relationship and how our mother tells her to leave every time they have a minor disagreement. She then said that when they fight, they each yell a bit and sometimes she hits him "but that's it". As soon as I read this, I knew I had to talk to her about it, but she had to go before I could finish working up the courage.

What makes it hard to talk to my sister about this is that her pregnancy is very high risk. She is going through A LOT of stress right now (finances, work, the usual life stuff) and our mom is constantly demonizing my sister's boyfriend even though SHE hits HIM (he has never hit her back). This is her miracle baby and I'm afraid she'll miscarry if we stress her too much. I don't want to make her feel bad about herself or attack her, but this is really not ok and I can't ignore it. 

I'm so sensitive to this because our mom had anger issues when I was growing up and would beat me for every minor screw up until I moved out at 16. She has apologized and is no longer violent, but I find it difficult to really love her like I used to. My sister desperately wants her relationship to work and I want to help her realize that her hitting is causing a lot of the problems they fight over. I have thought about asking her if she wants her (possible) future son to think it's ok for his girlfriends to hit him, but I'm unsure if this can be perceived as insulting.

NOTE: I'm not here to hear people insult my sister. I know her behavior is wrong and I AM going to talk to her, I just need advice on how to go about it.

Provide a brief overview of the text.
<bot>: My pregnant sister hits her boyfriend, need advice on how to talk to her about this in a few hours without angering her or making her beat herself up.