We have the same situation in our home. Our FTA placement just turned 2, and had been in his previous f/home for almost a year. C was his MOM, no ifs ands or buts. And the fact that he DID bond so well with her gives us hope that he can do it again.
The cw at first mentioned that *I* could talk to C, but there shouldn't be any physical contact between fs and C for a couple months or so. Adoption unit put a kaibosh on that, and said that she was to come see us as soon as she could after placement, and then after about a week, and slowly let the visits get further and further apart... until it's like seeing his favourite Auntie C.
I was absolutely completely terrified - or maybe insecure and selfish. I did NOT want her to come visit, I wanted to get on with "my life", but I now see how wrong that was. SHE is a part of his life, and she always will be. I can't change that.
He'd only been with us 2 days when she came on her first visit. I thought he'd scream and cry and cling to her, and want to leave -- he didn't? He did call her mum, and that was fine; he was also doing that with me - I'm Mama, she's Mum. She HAD been his Mom for 10 months of his 25, y'know? And I let them have that. The first visit, I didn't interfere and just let them "do their thing" - the visit wasn't FOR me; it was for them to have closure. We didn't want fs to feel abandoned by the only mother he remembered.
She's since been here a few more times, and last time, he said "Buh-bye Mum C!" when she left!
It was adorable. We will slowly drop the "mum" and start using "auntie" C, but when he's ready.
I would suggest: Let the fm come to your house (DON'T GO THERE! Too confusing, our adoption worker said), literally as often as possible for the first week or two. It will be very uncomfortable for you, but it's not about you (and trust me, it took me awhile to figure out THAT part! LOL). Then make it once a week, then every 2 weeks, and slowly get it down to holidays, etc. They need to know that she didn't abandon them! They could very possibly become angry with YOU if they don't ever get to see her again, y'know?
Good luck. PM me if you want to talk. Fs has been here 2 weeks now, and is doing wonderfully! I do think the visits with C did help him realize that this is home now, but she didn't just "go away" either.
~M