Inappropriateness between siblings

So my kids (our son and my step daughter) we’re getting ready for their showers so they were getting undressed in their rooms. My step daughter is 9 in April and my son is 4. My husband found my daughter laying on the ground with my son jumping around and laying on top of her naked. My husband yelled more so at my stepdaughter, because she’s older, and told her it was inappropriate. My question in all this is “should I be concerned?”

I’ve never encountered this with either child. My 4 year old is into the whole “look at my wiener” phase and using potty talk - poop, butt, etc. which I’ve been told by his teacher at pre school and dr that it is age appropriate. However, it’s not my son I’m concerned about. It’s my stepdaughter.

Back story... my step daughter is exposed to a lot of inappropriate things at her mom’s house and sees a lot more than a 3rd grader should - drugs, living in and out of motels, Mom with multiple boyfriends sleeping over in the same room and sometimes even bed as my stepdaughter. I’m worried she might actually not be so innocent in the sexual knowledge department because of her exposure in her other home.

1. Should I say something to my husband?

2. Should I be concerned about the situation we witnessed?

3. Should I keep a closer eye on my step daughter that she doesn’t instigate something else with my son?