My niece put facebook pics up and I've been fapping to them since yesterday.
Are you joking right now? Why would you have an attraction to your niece?
I don't know it just happened
This is something you need to stop. It is wrong to feel attracted to your family members and especially so if they are children.
It's hard but I'll try
You're masturbating to a child, and that is wrong on so many levels that you shouldn't just try. If you're unable to stop then you may lose the respect of your entire family, because it is considered extremely wrong to masturbate to your family members-- any of them!