Worried Step-mother with a 4 yr old & 3 yr old

My husband and I recently received his kids in October 2010 from their mother (who didn't want them anymore). Since we have had them, we have noticed weird behavior from them.
               Just the other day, I was looking in my digial camera and noticed alot of pictures of them taking pictures of each others butts, and penis. It disturbs me because I have 2 kids myself (7 yr daughter & 6 yr son). The youngest (3 yr old) tried to touch my daughter on the butt(she had pants on) while she laying down on the sofa watching a movie. All I can do is put him in timeout..We talked to them when the camera thingy was discovered.
              I don't know what happen while ith their mother and her boyfriend, but that's only the beginning... The small one seems to have more angry problem about something, his even played in his own fecus (2 times already), his pooped in my shower while I gave him a shower. His hit me twice, as if he was an adult, his even fights and hit his father.
   My point of view, I think he has saw something at his mother's house...We have ask her and she states no way, but recently i found her profile on facebook where she is posing naked pictures of herself.
               we do not in anyway show anything we should not in front of the kids. Can anybody give us some advice? Should I take him back to his doctor and have him checked for sexual abuse? Is it to late to tell? I called to schedule appointment with psysc...but due to weather(snowed in) we could not go, now I'm having to reschedule. Your advice would be nice of you...Because I don't wont my kids to be put in this mess much. And I have told all of the kids, if someone touches you, come tell us!

[lil_sweet_thang]

yes take them both to the dr take no chances