Also @mehGustai 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?)
I'd like to address your side-note separately because that's probably the one you're (all) the most curious about. I'll give you the whole story.
It was all her idea. She came to me about a year into our (D/s) relationship, she already had a collar she was free to wear or not at any time. She wore that collar from the moment she came home until she curled up in to bed with me at night, every single day. She couldn't wear it in public because she has a relatively high-status job, so it was kind of incompatible. She told me she wanted something she could wear in public, something she could wear out in public for all to see, that she was mine, that she belonged to me and nobody else, without it being seen as that. Just something that only we know about, a secret taboo-ish something. This idea turned her on like CRAZY, really, really, really CRAZY. And to be honest it also made me tremendously horny.
I first suggested a ring like I still wear from my former Dominant(in case you were wondering she is fine with me wearing that by the way). But she didn't want a ring, she said and I quote: "the only rings I'm ever going to wear are going to be through my flesh or around my left ringfinger". Also, she wants to wait with getting married until we're both in our early 30s and we wanna start a family together, maybe. We're still young so we're taking our time. Anyway, a ring was out.
Giving what she said about piercings I wanted to suggest she get one, but she was way ahead of me and wanted to keep any possible piercings for special occasions (anniversaries, great accomplishment etc.) So I moved onto the next logical thing (you know with piercings and tattoos usually being done at the same place), a tattoo. We had a very serious and long conversation about something this personal and possibly permanent. We hadn't yet decided it was gonna be a heart but she wanted something with my initials in it, and I put my foot down. I said no. Not because I didn't love her enough or anything but because I didn't want to take the risk that if we broke up or worse, that she would have a permanent reminder of that. We had our biggest fight ever about it (it was bad, the worst week of my life).
I'm not gonna go into details about the fight because this post is long enough as it is and she doesn't want me too. She's sitting next to me as I write this by the way, correcting my spelling and grammar and ever so annoyingly caressing my neck. Let's just say she got what she wanted, she convinced me why, no matter what happens she wanted my initials on her body, for the rest of her life. And that's about it, three weeks later she got the tattoo, on the small of her back. It looks like the emoji heart on your phone, it's about the size of average fist, with my initials written into it in beautiful calligraphy. To strangers and friends it looks like an unassuming, average tattoo, same as many other people in a relationship might get. To her it wasn't just a physical experience getting the tattoo, but also a mental one. While she was getting it she says she felt that tattoo and its meaning being imprinted on her mind and brain as well.
As I said it was/is one the greatest statements of love I will ever experience. I don't think I'll ever understand what I did to deserve her.