my niece is 15 yrsold, and got pregnant by a 20 yrsold ... she told him she was 18 can they get married and what is the penalty for him for this

At a minimum it's Rape 3 which is a felony.  Off hand I believe it is a Class E which is punishable by either 1 1/3 to 4, or probation, or both.  It would also require sex offender registration.  I handled a case like this in Queens a few years ago and was able to negotiate a misdemeanor non-sex offense.