Choosing between my husband and a baby! 😭LONG READ

Ok guys I have no idea wtf to do! My husband and I have 3 children (2yr old boy, 4yr old boy & 4wk old baby girl) We're young in our early and mid 20's and talked about having one more kid later in life in about 5 years or so. I just gave birth to our youngest on the 20th of October and we're still adjusting to life with 3 kids. We plan on moving at the beginning of the year and trying to get our life back on track and we have a pretty good plan worked out. We're not doing super great right but we know next year will be better for us so we're looking forward to it.

Now the problem!

My twin sister ended up pregnant unexpectedly and has been a rollercoaster of emotions about it. She never wanted kids and this happened from a one night stand so she's looking at being a single mom. At first she wanted an abortion and then decided not to go through with it and then she changed her mind and was going to get one and then changed her mind again. I get it. My sister and I are 21 so I understand the stress of not knowing what to do with an unwanted pregnancy. She ultimately decided to keep the baby and she's now 5 months along and is due in April. My sister called me last night and began telling me about how bad her depression has gotten(shes been dealing with depression since our early teens) and how she feels like mentally she isn't capable to be a mother right now. She said she loves her baby but right now she doesn't think she would be the best fit for him. She asked me if I would be willing to take legal guardianship of her baby. Now this is something my husband and I spoke about in the beginning of my sisters pregnancy because we felt like she wouldn't be able to handle it and we were afraid that she might not stop her partying lifestyle to be a good mother. She's stopped drinking and doing "recreational drugs" finally but she still feels mentally she isn't prepared for this. My whole family knew that she might end up asking someone to take the baby because she always made it very clear that she never wanted to have kids and this pregnancy was not planned. When it was just a "what if" situation at the beginning of her pregnancy my husband said we could raise her baby if she asked us to. But now that she's actually asked he's pissed and doesn't want to. He is pissed and feels like she needs to suck it up and just take care of her kid. I can understand that, our only planned pregnancy was our daughter so with our sons we just had to suck it up and figure it out. But at the same time I don't want this child to end up in a possibly toxic situation. I don't think my sister is super irresponsible or anything but she has said many times although she loves her baby she doesn't exactly want him and now she's worried she can't handle motherhood. She has made it clear she doesn't want him to grow up the way we did with a mom who has mental issues and really no real interest in being a mom when it's not convenient for her. She says she'll support him financially and she'll see him often and she still wants to be a part of his life she just isn't ready to be a full-time parent. She also says that it wouldn't be a forever thing. She doesn't want him to go to foster care or be adopted to strangers she wants him with family. I can understand why my husband feels the way that he does but I also want to help my sister and I feel like I have no choice but to help her. I'm so afraid of my nephew ending up in a mess so I'd rather he be with us until she's ready. I get that it's a big ask but I can't tell her no. I don't want her to put too much on herself because she's already at risk for PPD and she has a history of suicide attempts. My heart won't allow me to say no and leave this baby to go into the system or watch my sister fall apart or not care for him. She's been so up and down with this pregnancy sometimes she called the baby a parasite inside of her. She doesn't even really like to talk about anything pregnancy related She's not mentally ready for this but she doesn't want to lose him forever but I don't want to lose my husband behind this. He says he doesn't want to raise a fourth kid right now and I get it. I told him that it is possible that she change her mind before he's born and maybe she's just feeling overwhelmed right now. But she sounded pretty sure so I'm not really holding my breath on her changing her mind I feel like I'm stuck I want to help my sister but not lose my husband and I have no idea what to do! 😭😭😭😭😭😭😭