Thanks, Punkey. It's a crappy feeling to have to unravel this on someone else's terms.
Other mom (OM) knows that Aunt sees Mo but she has not spoken to OM about putting them in touch with each other.
From what Mo said Aunt told him "Oh I'll just call her and let her know that I told you to write to her on Facebook." I'd really like to write to her before that call happens because I don't want OM to think I was being so weird about things because I think it's wildly inappropriate too.
OM does not have custody of the sisters. They live with their father (who is not Mo's father). They have met Mo but it has been years (maybe 6 years -- so half his life ago) since he's seen them. I don't know their father or step mother at all so I'm not in a position to facilitate a meeting. I've told Mo that when they're all adults they can seek each other out.
Yes, I know he'll always have abandonment issues. He was very clingy last night -- unusual for a 13 year old boy.