just a little advice..

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so i wrote a while back on problems w/ my sisters, mainly focusing (at the time b/c there are always problems) on 2 of my sisters; one sister (sisB) was seeing a guy who my other sis (sisA)had already been with; long story short - sisA had a fling w/ boy; sisB meets boy they start talking.. blablabla.. no one agrees that they should be seeing each other b/c sisA already has been w/ him, i think its a little on the 'u just dont do that' side you know? ........ anyway... sisB tells me this AM that she & he are official; since a week ago. she 'forgot' to tell me.. we live together, u just dont forget to tell someone you live with that you are in a relationship; she did tell our mom the same day though.. i dont get it; i think she was trying to find the perfect time to tell me b/c she knows how i feel about it. now my thing is, sisA and i are a lot closer than sisB and I... should i tell sisA??? both of them have not spoken since about october (which is when i originally posted something) not one word! so i know sisB isnt going to call up sisA and tell her the 'good' news.. am i supposed to tell her? sisA feels hurt & betrayed by sisB b/c there are just certain lines you dont cross and she feels sisB has crossed those lines (this is not the first time they've had issues) i dont want sisA to find out when he shows up somewhere w/ sisB or anything like that; my family knows that i dont say much, i keep my mouth shut w/ just about everything.. so if something goes on w/ other ppl, i dont tell anyone, i feel if they wanted others to know, they'd tell them themselves you know? but this is different, i dont want sisA to get caught off guard so i want to tell her; but i dont want to start telling her my other sisters business... so i dont know what to do? i dont know when/how sisA will find out.. i dont want to talk about sisB behind her back, tho i wouldnt be talking bad. but sisA & i are closer, i think she should know. personally i feel she should have been the first to know b/c the issues involves her, sisB and this guy.. who is the cause of the problem....... any advice?