Me and my boyfriend has been together for 2 years now and I thought we had such a strong connection but I was wrong because for the past 5 months he's been helping my niece with her problems and he said he has fallen for her but I don't know what to do. He's the best guy I've ever known and I don't want to let him go. But I just want to know if there is a reason to kind of separate him from her. She doesn't know how my boyfriend feels and I don't think I should tell her because it would make things more awkward. So what is the calmest way to settle this? Me and my boyfriend go to church together so it's not like I can't not see him cause even if we did break up I would still have to see him 3 times a week at church

Falling for your niece is no crime, but make sure you both define boundaries of good behavior, and identify lines that shouldn't be crossed. This is between you and him, and he is the one accountable for his actions. If he cannot control himself, don't ignore that fact, and limit his activities with your niece.

You don't wave a piece of meat in front of a dog, and expect him not to go for it. SO, is he a DOG, or is he a MAN?

But I don't understand what I'm supposed to do

You talk to him, and set boundaries. I don't think it's a good idea to have him helping your niece when he cannot control his own feelings and you need to tell him this.