My wife and I are considering adoption , but she has a son from a previous relationship who lives with us (age over 18) and has a felony conviction will this affect our efforts to adopt?

Yes, it would effect the adoption; even if he were to move out, he is still part of the family and that would jeopardize the process.

It depends a lot on WHAT his felony was, whether he is still living with you at the time of the adoption, and what the laws in your state are.

I suggest you talk to an agency in your area to find out how MUCH it will affect the adoption.