As a mother, it is hard not to want to protect your child, even if they are grown. I believe the key is balance and not approaching it in a judgemental way. I think what others have posted is correct- about approaching her without the SIL and not coming with the tone of "look...I want to talk to you about something." The truth is you are concerned and I believe if that sentiment is expressed, then the concern is what will prevail. But, if you get the sense that she doesn't want to go there or have the conversation with you, let it go and simply let her know you are there for her.
I would also be careful not to speak poorly of her husband or say anything that seems as though you are judging him. Honestly, if you are able to have a conversation, I would let her know that since he is part of the family that you care about him and are concerned for him as well (if that can be a true statement), while also letting her know that she will always be your first priority though. Basically, make it clear that you don't despise him because that will be important to her to know that even if something is going wrong, that you can see past it to still love him and receive him in the family, while at the same time making sure she and her children are safe and keeping them the priority. When I say "loving him" I don't mean letting her think what he did is okay...because the love can still be there while completely acknowledging that what someone has done is not healthy or acceptable.
As mothers we already know, mother's take on a lot of responsibility...she could feel guilty for being in that situation...allowing her children to see and experience that...choices made, etc. So, for that reason, I say to just listen and you will know what to do as the conversation takes place.
I hope the best for you, your daughter, and family.