M.K.
Please know that there is no judgment in my response. I have not walked in a step-mothers shoes but I have had two. The first one did not consider my sister and I to be any part of her family just simply my fathers baggage. Needless to say that she and I did NOT have a very good relationship. Step-Mother1 caused many many problems in my family and hurt many many people. She also cause many issues between my father and I by making him choose us or her. 10 years later my father divorced her and finally re-married a woman who to this day treats my sister and I like we are her own. My father died 3 years into their marriage and yet 6 years later she is still very much a part of my life.
Being a step-parent is the HARDEST job in the WORLD, way beyond parenting because of the fact that you have all of the responsibility with out any of the clout. Can you take this boy into your home and love him as your own? I hope you can. It seems to me that maybe life with his mother is not very stable (any mother who would send her kids away for some man doesn't seems very capable to me). I understand your point about his mother should have arranged for him to see his brother but in times like that I think you have to be the adult...would it have really been that big of deal to have his brother over? How is that much different than any other friend? Also try and put yourself in your husbands shoes, would you worry about it making sense to let your child move in with you?
I would suggest having a serious talk with the hubby and making some lifestyle changes, ie. "going out a lot" so the work doesn't. If you guys do decide to let him move in..you may do some family counseling. It will be a big adjustment for everyone being in a new town. Good luck and no matter where he lives please for the sake of all of your children remember that he is just a product of this situation not the cause of it. He is a child and you are the adult. It's tempting and very difficult I'm sure to ensure that step-children are not punished for mistakes their parents made a very long time ago.