I don’t know the fandom, but...
If you’re after keeping the situation fairly platonic and familial, then that’s easy enough. You can do that by way of what the sisters notice about each other while they’re snuggled together. As the author, you provide the cues for the reader. If, however, you want the situation to be erotic for the reader, just not for the characters, that’s a slightly more tricky proposition.
In that case, I’d suggest pulling the narrative back from the characters a little at times, so that you can be quite descriptive about what’s going on physically, without needing to involve how the two sisters feel about it, as such, and just dipping in here and there to peek at them enjoying these moments as sisters.
If, on yet another hand (is that three so far?) you want to have a kind of charged erotic situation with some unresolved sexual tension going on, well then you’re probably best off concentrating on one of the characters, and going deep into them. Letting them have all their dirty thoughts, while trying on the surface not to let it become apparent in what they do. Particularly focus if they feel any guilt or shame. Those emotions are always good, especially where incest is concerned, as characters will generally try to rationalise it for themselves.
If, on a fourth hand (and at this point I’m probably using someone else’s hands), the sisters are already in some kind of established incestuous thing and they’re both cool with it, but for some reason they’re not going to start eating each other or something… well, then I really don’t belong on this thread… *runs away*