If you know that Adoption or fostering isn’t something you would be happy with, it sounds like you know in your heart what the right thing to do is OP. 
I don’t want to minimise any of the problems your Dsis may have but being mute at 2 and a half isn’t necessarily indicative of FAS or other SEN but as I say, you will know her issues.
It sounds like you really care and want to give your sis and your other DC the best, but it may be that you are over-thinking this a bit.
My gut is that this is a 2.5 year old who already has an attachment to you, which it would be damaging to break. Think about what message you would be sending to your other DC if you didn’t step up in some way. Maybe you & your other sister could do some kind of shared care.
The fact that your DH wants to take her too is significant. She is wanted and loved, but you are understandably nervous. I don’t want to do the fahmlee cliche but I do genuinely believe (and I believe the studies bear this out) that kids fare better being looked after by those with a biological link which is why SS are always keen to facilitate/investigate this first & foremost.
I’ve got 5 kids, some of whom were not planned and came at a terrible time for us. Personally, abortion could never be an option for us, and I was in despair about whether we would cope. We had no money, a tiny 2 bed house, DH had lost his job and 4 babies under 5. I seriously considered adoption. It was far from ideal. I had serious anxiety & depression & didn’t think I could do it.
But a few years on, I earn good money & life has turned around. I understood all that went with children & really didn’t think I could cope with the sacrifices and that it would be unfair on the children already born in terms of attention etc.
Actually it’s fine. Hard at times, but fine. The children know they have to make sacrifices as being part of a large(ish) family but genuinely no-one would be without each other.
You could find that it works out fine.
Don’t feel bad for being realistic/pragmatic but also be aware not to project adult feelings onto your existing DC. They’ll have to step up and help out, which happens in lots of families, especially bigger ones or where there’s a crisis. I think you sound much stronger than you give yourself credit for.