I would stay out of it! Last year my soon to be BIL did something that I thought was *cheating*. I thought my SIL should know about it (because I would want to know if it was my fiance doing those things). So, I told her. I also mentioned my feelings about what he did. I should have kept my mouth shut! My in-laws and SIL are still mad at me...lots of tension in the air over it. I was accused of trying to ruin their wedding,etc when my real intent was just to say "hey, is this ok with you because he's doing it and I would want to know if I were in your shoes". They took it all out of context and our relationship will never be the same sadly. Sounds like your sister would be one of those people who would take your *concern* as a slap in her face and that you are "siding with mom". She may black list you as well. From experience, let them work it out unless you want to be put in the middle and blamed for whatever happens

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Proud to say I haven't shopped at a Wal-Mart since Sept 2003