Wouldn't she have mentioned it, along with the claims of abusive and controlling behaviour she already listed. I mean, she said he strangled her once, didn't she, and she has said that she had to ask for money from him .... so if he was forcing her into sexual activity in return for things, surely she would have mentioned it? It would help her defence for the jury to think he was doing that kind of thing, so why keep quiet about it? Especially as she was asked specifically about their sex life, so an ideal time to disclose it, surely?
As its hearsay I just don't think we can know. It's impossible to know the context. Was SH expected to give sexual favours to NM in exchange for want she wanted ? Was it just a joke and it never happened? If it did happen, was she groomed to think this was normal? If it happened did she instigate it and think it was cool?
Also the stepmum said this was when she was 16 which is five years ago. If it was a joke then maybe it was one she had forgotten.