Sorry for the long post...
5 months ago my eldest DD (from a previous relationship) moved in with us (along with her two dogs). Her long term relationship had broken down and she had been on antidepressants for about 6 months prior, since the relationship began to deteriorate.
She tried initially to move back in with her dm but the household was so volatile that she felt unsafe. There were regular arguments between my ex and her bf often resulting in physical violence and when my DD tried to intervene it would make things worse. As an example, my ex would threaten to crash the car into the house to kill them and would tell my suicidal dd to "jump off a bridge", it felt like a waking nightmare and all I wanted to do was get her out of the situation.
We had tried from the beginning to convince her to come and stay with us but she was worried about how her dm would react (an understatement to say the least) and thought she and her dogs would be too disruptive to our existing household - we have 2 DS, a DD (my DSD) and a cat.
My dw suggested temporarily rehoming the cat with family so that my DD could come and live with us and had somewhere stable to get her mental health back on track. We converted one of our downstairs rooms into a bedroom and she moved in. For me it wasn't just a weight off my mind that my eldest was here being looked after, having all my kids together meant the world to me and I was really happy (there is a fairly big back story about the early years of my relationship with my eldest, eg multiple court cases due to multiple broken contact orders, so her living with us was a huge deal for me).
The problem is that since she has moved in my DW has seemed to resent it. At first she made comments about how happy I was, but would say I should have already been happy with her and our kids. I was, of course, but given the back story, i don't think its unreasonable to feel happy that my eldest was with me at last nor does it mean I don't love my other kids.
Then she started making comments about the dogs, how she hated them and hoped they'd escape and get run over (something totally out of character for her and something nobody would ever think she'd say in a million years).
Then one day she told me she was thinking of leaving. She said the dogs made her so miserable and that even though we had taken them in with my DD to make her happy, she was entitled to be happy too and if that meant leaving she would. I just didn't know what to think. I felt like I was going to lose my DW and other DCs because of some dogs! If I'm honest I thought my dw was being quite horrible but I know I am probably not seeing things objectively. I fretted over my eldest thinking she had split the family up if she found out why. Later my dw said she didn't mean it and things were OK for a while.
Lately however things haven't been so great. She wants the cat back and that can't happen while the dogs are here. We could tell my DD to rehome the dogs but I think she would rather move out and I worry how she'd cope on her own (her mental health has been very bad since the break up and the issues with her dm). I also feel like I'd be letting her down as her dad if she ended up moving out. If she stayed but got rid of the dogs I think her mental health would plummet. My DW says all I care about is my dds happiness and not my DW. Obviously I care about them both but I feel like whatever I do will be wrong. I'm worried that by trying to do what's right for my DD I'll lose my dw and other dc!
I couldn't honestly imagine my dw giving me the time of day if this situation was about my dsd. If I said I would rather have the cat back than my dsd live with us I'd expect to be told to fuck off. Part of me is angry about it but if I said that I get the impression my dw would just take the kids and go.