My vote is
Amelia for the ex. The outgoing/sexually experienced sister having an ex is almost too common a trope, and I think Amelia having a girlfriend at one point and then breaking it off because of choosing her career, or the career becoming too much a focus could be an interesting path to go.
Plus, she broke it off before "sealing the deal."
Maybe that could also be a reason, and then Kate wanting to claim her also leads to Amelia finally sealing the deal?
Could also have a stronger NTR point where she loses her virginity to Kate on this specific path, since maybe she wasn't too into sex at the time before and never did anything too lewd?
Don't know, but to me is seems like there are more story options with Amelia.
And yeah I do kind of like this level system with the sisters. The two older sisters are most veluptuous, the two middle sisters (including MC) are still juicy but not as much, and the two youngest are the least, emphasizing the petiteness.
And
and yeah, I like my sharing and NTR with futa x female. Whenever there is an option for it, I'll be there!