This could be an overpair as easily as a made straight, although its hard to say for sure since the BB got to see a free flop and can be holding literally any 2. With no reads it's hard to put him on a hand at all but are you really going to lay down a flopped set here? You limped the PP specifically to hit your set and you did so I wouldn't fold just yet. The other question to ask is WTF is MP2 calling with? Based on his postflop action I think his most likely range is an overpair to the board and not a set or a straight, maybe even A3,A7, A5 (with A5 being rather unlikely) and if either of these two are donkeys they could easily be overvaluing any Ace.

With about 5k in the pot and 800 needed to call you're getting the right odds to draw at your 7 outs to a full house or quads, and if the turn bricks you pickup 3 more outs for a redraw on the river. The problem here is MP2 and whether or not you're going to end up sandwiched in between another raise or, even worse, someone pushing all-in. Based on the fact that MP2 didn't reraise you originally I'd say that if he is going to hang around at all he's probably going to call again and not raise.

With all that considered I would probably call here and push any safe card (any non 4 or 6) on the turn. If MP2 raised and there was more action on the flop I would push right here and hope that one of the two will call holding an overpair or on a draw. If someone really did spike a set of sevens or straight on the flop, so be it.