Look, to my mind, you're better off sticking to the rule that any sort of sexual contact with people under 18, sexual defined in terms of whether you get turned on by it, is probably better to avoid. If nothing else, you'll feel a little creepy about it. And what's this about playing with her aunt's foot or something? Look, to my way of thinking this is disturbing. I think the test is this: if you get turned on by playing with her feet, then don't do it unless you would also be fine playing with her breasts.
I'm interested in the point before, in part because I think it is mistaken. The idea is this: right and wrong is pretty much subjective, so if someone says what you do is wrong that's just their perspective. The trouble is that from a pragmatic point of view disagreeing with people on fundamentals does you no good. To put this another way: you may not see anything wrong in a particular behaviour, but if the other 6 billion people in the world do then you either need to rethink your position or come up with a way to convince them of the legitimacy of your way of thinking. And I don't think the old relativist line will cut it. To get even more techincal, surely "wrong" is a concept that relates implicitly to other peoples' ideas - you can say you approve or disapprove of something, or that other people, given their background assumptions, should agree or disagree with it. But in the end, someone who is excluded from society is a pariah whether they're right or wrong from a logical or pragmatic point of view.
Point is, I'd be very careful doing this sort of thing with friends' family members. However you justify it, the fact is that it will generally be seen as kind of sick.
Just my 2c.
Cheers.