Who woulda thunk it! Already a prime opportunity to use my new found skillz for winning friends and influencing people!
It's about the dog.

No, I'm not trying to influence the dog.
Here's the deal: The original owner would like her back. The original owner is D, the already-ex-brother-in-law. He was the one to choose Sadie (the dog) as a puppy, pay an obscene amount of money for a dog (but I think any money for a pet is obscene,) and spend a lot of time training her. Because of all that, I have no problem calling him the owner.
There are a couple of problems though. Namely the stbx and the stbx-sister-in-law. Both of them think the dog is theirs. Not individually, but 'theirs' as in, "not anyone elses."
The stbxsil and D had a horribly nasty divorce. One that's lasted 5 years now, and is still going on. Somewhere along the line, it seems the stbxsil got 'custody' of Sadie. I have no idea if it was a legal precedent or just something that happened that way. At another point, further up the line, the stbxsil couldn't keep Sadie. If I remember correctly, it had to do with her housing situation. "No dogs allowed" kind of a thing. So we took her in. I'm taking care of Sadie now because my stbx can't feasibly have big dogs. I think she's allowed to, but it's quite a bit more rent, not just a larger deposit. Besides, it's a little 2 bedroom apartment; not exactly ideal for a full grown golden retriever. So I'm housing and feeding her.
I don't want a dog.
Sadie's a great dog. Loveable as can be. Great with the kids. But I don't want a dog.
D wants her back. Now's my chance to send her home. But those 2 obstacles need to be overcome. Right now, my thought is to try and convince the stbxsil to 'approve' it. If she does, that totally removes the stbx obstacle. In fact, the stbx would only be an obstacle if I went to her first.
I haven't spoken to the stbxsil in 3 or 4 years. We left on ok terms, so I'm hopeful that I can come up a with a convincing argument. I think I'll talk about how their daughters will be getting Sadie, not just D. D has custody of at least 1 of their 2 daughters right now. The other is in a therapeutic group home but will probably end up going to D.
So I think if I use that tactic, that Sadie will be going to their daughters, there might be a better chance of it happening without hurt feelings or repercussions for me. We'll see.