Seller financing, when does the seller realize gains for taxes

[John Bartlett]

Hello, 

I have two family members that are 50/50 owner on title of a house (one lives in the house, one does not), they would like to sell the house to a 3rd family member, and seller finance the sale. I am trying to see how the sale would impact the two family members income, and when realized gains would impact them. I know blog post aren't always the best, I'm more looking for a direction to look, or a tax professional that fully understands seller financing. 

thanks