I agree, but I'd be kind of surprised if the LW were able to deconvolve the reasons for his/her suspicions. That requires a lot of personal insight. Telling the LW to get lost would be a totally reasonable reaction on the part of the sister, assuming the suspicions are false, but I maintain that it's a risk the LW has to be willing to take to protect his/her child.

If the suspicions are false, we have an adult making some ugly assumptions about a sister who will have to decide how to deal with them. If the suspicions are true, we have a child victim of sexual assault. I appreciate that the first scenario is mean and gross, but seriously, the latter is so much worse, so the LW's priority has to be to investigate and protect the child.

If the LW is smart, s/he can talk to the son or sister about the hasty dressing and leave the sister's past as a dominatrix out of it.