[Larude]

A pair of nines is a nice hand which you need to raise almost always if you play it, especially in middle position. This is a valueraise. Moreover you don't want all sorts of crappy hands come in there, you want to have position on MP1 and you want to get CO out. Not raising in this spot is a fault IMHO. On the flop you have to pot it, you don't want anybody to draw cheaply to a flush. On the turn nobody has shown real strength, MP1 could think he can take the pot cheaply with a semibluff or he is raising with for value with a nonflushhand. I would call here and evaluate the river.