I dont think the bosom touching is a problem a lot of children do that, but its not great he is asking his sister to take off her underwear ,maybe he has seen some behavior somewhere it does sound as if he is copying. So check out for magazines at anyones house, inappropiate TV or PC it may not just be your house could be anyone elses, did you ask him, very often they just say they dont know... so be observant your self. Tell him its not a good idea he asks her to do that , then be aware but dont make more of an issue of it.