Advice regarding stepkid violence

So I was not there for this, just to clarify:My SD (8 yoa, 60 inches tall, just shy of 60 lbs) had a sleepover at her grandmother's because her cousin (nephew, same age, a little smaller in stature) was in town for the weekend.DH went to go pick her up this AM, as well as fix a curtain rod that nephew broke. SIL takes cousin to a museum, but SD couldn't go because her mom is picking her up early.When SIL and cousin left, SD throws a huge fit. Repeatedly hit my DH and MIL, to the point that they locked her out of the room and SD ended up screwing up the door. It has taken over an hour for her to calm down.Just let me say, I feel for the kid. She doesn't have it the easiest because her parents are divorced (happened when she was 2). Her mom is in school, so SD gets bounced between her mom's and her maternal grandfather a lot. I suspect there is not a uniform way that discipline/boundaries are being laid down between three houses and school. She usually get teary when she and her cousins have to say goodbye, but this is a whole new level.I'm currently 17 weeks pregnant. This behavior concerns me because I don't want the baby, myself, or my husband's safety to be in jeopardy. Husband is a big guy, but I'm pretty petite, and obviously, this baby is going to fragile for a long time.I don't know what to do, I don't want to come between DH and his daughter, but this freaks me out. Any advice would be appreciated. Sorry this was so long.Edit: Grammatical fixes via /r/Parenting http://ift.tt/2dTouKY