I don't have any first hand knowledge - none of us on here will, so definitely you will need an attorney.
Only you know your family dynamics. This would be a disaster in my family. My sister balked when I dripped wine on the shirt she loaned me to wear for the day when we had a change in plans one trip while traveling. For me to raise "her" baby would be a nightmare.
I would think it would be MUCH trickier navigating the emotions involved of having the bio mom still in the family. BUT only you know your sister and the relationship the two of you (and the rest of your family) have, so you can gauge what fallout there might be.
If it were me.... I would treat it as a "normal" adoption. So, the "fee" would be the same.
You didn't ask this.... but a single 42 year old friend of mine did artificial insemination. BY FAR the best of all worlds. It's all her baby - no messy issues to deal with.
Good Luck.