OP, I'm childfree by choice, and honestly, I wouldn't do it if I were you.
It's clear you're already feeling resentful at even the thought of it-this is likely to show through if you do take them on, even if you do your best to hide it.
I'm so tired of seeing the posts trying to guilt, OP (thankfully not the majority), or make out that there'll be some amazing, hidden reward in it for her-really? "Oh my god, I know you didn't want kids and your sibling's and their ex-partner's fuck-ups have upended your entire life-but do you see that very faint, blink-and-you'll-miss-it silver lining on that cloud in the distance?" "No? It's there, trust me-you may need to squint, but it's there!"
It's a sad, tough situation all round. However, you need to remind yourself that you haven't put the kids in this situation-their parents being shitty parents have. That's not your fault or burden to fix.
If you're not 100% in, you're 100% out. It wouldn't be fair to take on kids that you are secretly counting down the days until they leave and loathing every second of the hindrance they place on your life.
I think if you wanted to do it-and were fully open to it-that would be a great, wonderful thing to do-but it's clear that you don't. I'm just of the mindset that kids are an all or nothing kind of deal. And these kids will come with a whole host of issues from their parents that you may need to deal with. Along with tensions in the family from the sibling and their ex, dictating to you and likely placing blame on you.
10 years is a long time to be putting up with this. A very long time, indeed. For the simple fact that you'd have to facilitate all meetings between child and parent and all the complexities and crap that comes with that-I'd say no on that basis alone.
It's important that you make the choice independent of guilt, misplaced feelings of obligation, and pressure-it has to be something that you want and are willing to do. SS will do their utmost to guilt you into it-but if it's not right-say no.