Y&R 2/18:I think we know who Mama Bear is...
Based on Jana's horrified reaction to the identity of Ryder and Daisy's mother, it's got to be Sheila. She's the only one who would envoke such a terrified reaction from Jana. And Jana was genuinely shocked, which makes sense because Sheila is presumed dead.
The only "twist" I didn't see coming was Ryder and Daisy being twins. But this actually makes sense because it was getting ridiculous to imagine Sheila having truckloads of children (Mary, Diana, etc.).
I'm not sure if Y&R is going to try to say Sheila had Ryder and Daisy BEFORE she stole Scotty, but that would annoy a lot of people because Scotty's been shown to be older than them. Well, Scotty looks older than Daisy but he and Ryder could be about the same age. Daisy would just have to be playing older. The only other explanation would be to say she had these kids between her stints on Y&R/B&B.
My fan theory for Sheila's return YEARS ago was that Jana was actually Sheila's rapidly-aged daughter with Massimo from Y&R. Since Y&R ignored Sheila's children last time she returned, I wouldn't be surprised if it turned out Sheila was using a "stand in" baby to pose as her child with Massimo in 2002 and didn't actually have a kid with him. Sheila's daughter with James Warwick disowned her, so I don't think we'll ever see her turn up on Y&R.