i disagree with that. it is completely possible to be in love with more than just one person. sure more than 2 will definitely push the limits and that these people are your bloodrelatives by chance, won't help either. but love knows no boundarys and it's up to each on their own to follow those feelings.
as
MauritkCL already said, everything is optional. so you can choose if your MC is love-driven or an horndog. oooorrr you could play him as love-driven horndog. because not being able to keep your pants up and loving people with your heart, are not exclusive to each other (it's just very difficult to build a relationship around that premise)
true, the title "sisterly lust" is almost impossible to seperate from incest, but only almost. as it could be interpreted as that the 3 sisters share a familiar desire for sex, sexual selfexpression and lust in general.
so regardless of which sister you choose, there is always the same "sisterly lust" you will receive (but yeah, if you have a problem with incest this game should be avoided)
what
MauritkCL said is already good input, i would say, but not necessarily everything. i do say, that the part about "the talk", should be really focused on!
i assume (i do not know) your problem with the "bella sub"-route,
Perverteer, is that despite there noone gets really hurt, it's also not really consent between both partys (at least that's what bothers me).
bella is always shown annoyed by MC's demands, but then "shows" her approval by having orgasms or docile execution of the demand. now i don't want to label this anywhere near as rape, but an orgasm isn't a sign of consent, neither is following orders. to me it seems the MC holds all the power over bella, without her having any security measurements and that's extremly dangerous for any sub/dom relationships. i am not saying she needs a safe-word (that's more for the hardcore stuff), but there should be made some basic rules on how to go about and she should show actual enjoyment on being dominated and or talk about what she liked about it (like: "i cumed when you turned the vibrator on during my shift and it was awesome!"). that being said, she shouldn't go completely horny-mode and remain a bit feisty, as that is one of her major character feats.
what i personally would like to see, is bella as cuckquean. and with megan we would already have the perfect cuckcake, as with the shop scene we pretty much entered those waters.
i'm talking about bella helping MC hook up with megan, stand watch while they endulge with each other, or straight up use her help during sex to give megan some waves of orgasms while bella has to make do only with her or MC's fingers.
but i guess that's more of a niche-kink.
what i'd want and what a lot of people probably could agree on, is retaining the dom/sub status between MC and bella, but their interaction gets more loving. meaning that, MC could play a little with her feelings, but bella responds in honesty and MC overtaken by that, takes it to heart and gives from time to time some show of affection (not some more kinky stuff like another vibrator, but actual couple stuff). and bella shows affection through submissive acts on her own accord, without being demanded.
generally having bella being a hotheaded proud woman in public, but a docile snuggling pet in private.
that are just my suggestions, but it's needed that the bella-sub relationship gets some communication.
(sidenote:
MauritkCL your girlfriend has a tattoo as symbol of submission? that's really hot! does it have some hidden characteristics to show it's meaning, or is it a normal heart that you two gave meaning?)