My stepson won't help with bills I tried to tell him to leave but his mom sides with him I live on ss and can not support him and his girlfriend

If his mom is on the deed to the property or on the rental lease, there is nothing you can do to remove the stepson. Any attempts to legally remove him will require her approval.

So your only recourse is work with his mother to get him to contribute. Sit down with the mother and go over your finances. Show her that you don't have the funds to support him and his girlfriend. Show her how his being a deadbeat will affect her.

Offer compromises. Maybe put locks on the refrigerator and pantry. Allow him to live there, but not sponge off you. Restrict access to parts of the home, He can stay in his room, but not use other parts. She may go along with some of those.