I think the husband is putting his step children AHEAD of his wife. I am a bit concerned that these stressors in their life will break them apart. So many questions could be asked. Is it abuse? It was ONE incident you witnessed. Abuse is usually an ongoing thing with real mental and/or physical intimidation.
Here are some points to consider:
Not your business, you are not in the home, your view is limited.
Since there are "step" children, then we have previous marriages to consider....maybe he cannot keep a woman long? Who really knows?
Does she give as good as she gets?
Is this husband BOUND by a vow to a dead wife to care for these children or has he made some promise which you are unaware to see to the needs of these?
There is just too many variables I do not know about. If you are really, really concerned for this woman, ask her privately if she is doing OK in this relationship. You do not have to detail that you suspect abuse, she would just tell hubby and if that was a one time incident, they would be laughing at YOU for thinking that.
If I were you, I would express my concern privately to the woman and then let her know if she needs to talk, you are available. AND then YOU do not share all you know with everyone you know! Be a trusted keeper of the truth for this woman and let her kKNOW all info STOPS with YOU. Then hold yourself to it.