Sigh. I was the 8yo who took care of her little brother (4) after my mom left my dad. My dad of course had to work so I was in charge of my brother and housecleaning. Unfortunately, I still rebel and regress, sometimes, and I am 43. Certainly a "big sister" can help, but it's too much to be in charge.
I haven't read the responses, but here goes...
I am not sure what you can do beyond what you ARE doing, though. If you have any good relationship with mom, can you help her define what is an appropriate role for an 8 year old? Will mom get mad and that make things worse? Some regression is normal when a new sibling comes but that is complicated by the new families in your situation.
My 2yo "helped" when I had my son, but that was things like getting a diaper or reading beside me while I nursed.
When we had #3, my oldest was 9.5 and was and is a lot of help. She just turned 13. She also took a babysitting course. Sometimes she is resentful of siblings, but truthfully, they can be annoying. My 10 yo son (about to be 11) also gets irritated with the youngest (and his big sister, as a matter of fact). Some of that is normal, too.
I agree that leaving a 7yo in charge in the absence of adult supervision is not wise. I still don't leave the 10 yo in charge, except on dog walks.
Don't know what else to tell you, but wish you and your stepdaughter and her other family well.