Dude wtf? Just talk to both - if you are as you say talking normally to both of them and given that it is facebook they are both aware that you have them added to facebook and given that sisters talk they would know as well. Then consider the fact that according to you, you are talking normally with the first. So at this stage it is not as if you committed adultery. The decision you appear to be talking about should first be required when things go beyond normal conversation and then you just basically have to stand up and make a choice. But keep talking to the other, they are sisters - if it turn into anything with either then being friends with the other would never hurt.
Also before you even start to consider this situation you are depicting, you need to realize that the girl or girls are not considering it in the same light (I am still going by the "normal talk") so for them/her you are just a friend one can talk to - even if you talked (I assume wrote) a lot with her. There is a difference between talking with a friend and talking as lovers, and judging from your scenario you are not at the lovers point and thus the girl is talking to a friend, not a boyfriend and the same would be true for the newly added sister, for whom it would be quite strange to consider you a boyfriend. So in that case its two sisters both being able to talk to the same guy, whats the harm in that? I've spoken to several sisters at once, and even though we had a splendid time and talked a lot after, I highly doubt any of them considered it a beginning of a romance. Granted neither did I, but as a guy I could not help consider them with interest - an interest that may or may not be returned. In another situation I was with one of the sisters, but I still gladly spoke with her younger sister. The older sister paid it no heed, because why should she? I was primarily her fairly close friend, so me speaking to her younger sibling was just as if a female friend spoke to her sibling. No need to consider it a romance drama.
If you are so easily stumped by the situation you describe then I am fairly certain you love neither.
If you fear being treated as a playboy/cheat then what have you done to make that appearance? If you have done no more than to add the younger on facebook, why care??
Anyway good luck with it - and just consider them persons like yourself - not some absurd notion without a mind of their own to speak outside facebook or make observations.
