Woah, have you been reading my mind? My next writing project includes two twin sisters (both in early 20s) who are in a lesbian relationship together. SPOILER - One is a main character later, the other dies and her murder is a key plot point. Part of the development of these two though, and their interactions with other characters, includes how uncomfortable some people are to the situation, and how the main protagonist has already known them for several years so is at least used to the whole thing.
Anyhow, I don't know how I'd react in real life if this happened, since I don't have kids and wouldn't know until it actually happened. At any rate, my own thoughts on incest are a bit of a tricky topic and could be considered a bit controversial. In essence I think incest shouldn't be demonised PROVIDED it's between consenting adults only, and provided there's no children born as a result. So keep the rubbers on, people! That's where the problem is though, most people will just say "urrrrgh, squick!" at the idea of two relatives boning, whereas I see it as just another aspect of human sexuality. I don't want to get with any of my relatives, and definitely don't want to indulge in incest myself - the closest I've come is finding two girls who I have only met once or twice each, who are second cousins of mine, rather attractive. Nothing more. However, if someone else finds his sister to be attractive and nice and develops feelings for her, and she feels the same way about him, and they're both of legal age and able to make a sane and sensible decision themselves, then I don't see why they should be vilified for being in love.
At that point, then, the issue comes more onto the fact that it's two sisters or two brothers. In that case, I don't have any problems with gay people in any way shape or form. So it wouldn't be a problem in that respect, at least not as far as I'm concerned. Incidentally, before some people try bringing out the religious argument, don't forget there's plenty of incest in the Bible, not to mention the rarely-discussed topic of Adam and Eve's children...