Hello, and what a beautiful family you have!!!!
I think the key here is to use caution and calmness when you talk about these issues. If you make a huge deal out of it, you may get a reaction to your reaction rather than a decision based on knowledge and understanding.
As far as the neighbor boy goes, you can calmly explain to your daughter that you are uncomfortable with him being so touchy. You could tell her that friends are great when they are boys and girls, and they can do lots of fun things togehter. You would prefer that he and she keep their hands to themselves. =) That should be simple enough for her to hear your boundaries and not to be so intrigued by your reaction.
Your son is adorable!!! I'm sure your step daughter isn't conscious about how her feelings impact him and his feelings. What you can do is have a talk about families and what makes a family. Talk about the sharing and uspport, fun and laughs and working together.