I guess it sort of depends on what you perceived to be "massive challenges". Yes, lots of adopted kids have trauma related issues, and trauma is a weird thing that impacts each child in different ways. But you sound like you might be familiar with the concept of advocacy for a vulnerable person, unafraid of using your knowledge of institutions to ensure the best care for your sister and willing to explore different options for care, all of which could be useful.
That being said, adoption is hard. Ours was considered a "tricky" placement, and the fact that they were siblings multiplied this several times over. Not to go into too much detail, but it involved introducing the two of them to each other when they were older, not as one child and one baby. It was more difficult because they had the capacity to irritate each other and be aggressive to one another in a way that a baby couldn't, and although you don't think you blame one rather than the other, when you are on your knees with tiredness and at the very end of your tether, it is very hard not to wish very very hard that one would just for once stop hitting the other one... and also that the other would stop hitting back.
You have to be very sure that your BC can cope with what could be or become a tempestuous relationship, and the sharing of your attention. Also, it is easy to have an idealised version of siblinghood as someone to be united with, to suffer the slings and arrows of life with, but there is no guarantee they will like each other, or not resent each other, and as their mother you need to be able to sit with those feelings and support them both with it without taking sides. I can imagine it would be tricky for you to hear BC potentially feeling like they blame you for upending their life (not that this will necessarily happen) but you have to create the environment where they could speak to you about it just the same.
As an exercise, you could also consider whether you might begin to rule out adopting certain children based on sibling status or complexities- for example, would you consider adopting a child who was the 7th or 8th child and had other full biological siblings? Why or why not? How would you support contact if appropriate? Would you want BC to come with you if you met? Why or why not? Would you adopt from a younger BM knowing they they might have other birth siblings later on? Would you tell your AC? Would you tell your BC? At the same time? What if AC didn't want BC to know because it felt too private and raw? Would you adopt a child whose older siblings had ADHD and were placed with biological grandparents? Would you know what to look out for if you knew BM had "drunk a bit" during pregnancy? What if she had with other pregnancies but swesrs she didnt with this one? Although this may seem very specific, discussing these possible situations and what you would deem to be "acceptable " risk might help you feel clearer about what you can and cannot handle, and there is no shame in being very clear about that.
Also, think about your worst, busiest typical day, and your capacity and how you would parent on that day. Then look through these boards for some indication of the types of ways that trauma manifests itself and think whether you could parent theraputically on one of those days. I hope I'm not being condescending, but trauma isnt convenient or weeping silently at bedtime. Lots of the time it is noisy and angry and making strange noises or growling at guests at a party. Many of my posts have detailed the weird ways trauma has manifested- we don't wear heavy shoes on long car journeys because we throw them, one of ours ate a tube of pringles for Christmas dinner, one refuses to talk at nursery.... nothing on its own that is completely unmanageable, but wrestling a hungry screaming toddler with a biting incident report in one hand, clothing he has deliberately peed on in another, into a car seat after he hasn't said a word at nursery all day only to set off and have him lobbing his trainers at your head is genuinely difficult to handle. All trauma, but a far cry from an easily soothed sad child may people picture when adopting
All that sounds very negative, and it isn't meant to be, but it's important to be realistic. Your own mental health can take an absolute battering, and if you have other caring responsibilities I can imagine it would be almost completely overwhelming. I was as free as a bird apart from owning a cat, but over the last year lots of other stuff just faded into the background. It had to so that I could care for the kids who needed me so much, but if you have other stuff you have to prioritise it can be very hard to balance.
I would say that our journey has been, as many others have said, a rollercoaster. Our kids are neurodivergent, its unclear whether thats trauma related or not, but the parenting strategies at their ages are relatively similar. Some months are good, some are bad. We are weathering them all out one way or another. We love our two little chicken nuggets beyond all belief, and yes, their neurodivergence makes some of the stuff they do a bit weird and wacky, but it also makes them absolutely hilarious and clever beyond all measure. If you have neurodivergence in the family it may well be that they slot right on in. For example, for some funny old reason, our whole family has benefitted from the use of visual timetables, ear defenders and weighted blankets, not just the kids.
It is not an easy choice, OP, but do your research, find out more and try to go beyond the surface ideas and into the specifics. There are loads of people on these boards and on Insta who give a very honest and real life understanding of what modern adoption might entail. It is by no means all doom and gloom, but it isn't a bed of roses either, and genuinely thinking about what you, your BC and your family unit can handle or want to achieve will be super important going forward.