Hey Mitch,
Sorry to hear about your plight. It is often hard being the step mum, to have to deal with the issues with a slightly removed sense of authority and control. I am sure you want what is best for yourself and your family, and sometimes it is hard to juggle the demands of others that aren't as closely related to you, and manage the guilt that comes with it.

I would suggest taking some time out for yourself to consider what YOU want out of your life, and what is best for your family. Easier said than done? Of course! Before making any permanent changes, it is best sometimes to wait and see what happens. You sound like you weren't expecting SDS to lob up on your doorstep, and it may be that his mother will come back and get him just as suddenly. Before making any permanent changes and adding on rooms etc, just wait and see what pans out. Also, remember that when children are little they have very few needs. After all, I shared a room with my sister until I was 11, and it never bothered us. Therefore if you feel you would like another child, then concentrate on what you can change, and not worry too much about all the stuff you can't.

Sorry to rant, but I just wish you the best in your dilemma,