Hey Jezzy... I haven't been around much, but reading your last few entries... sounds like you're in the same place as I am, you're doing so well, I'm proud!
You are right to point out your sister's downfalls - snacking on things, giving the excuse 'they're low-cal, so it's ok!!' isn't good enough. You are correct in assuming that it's a very slippery slope. .. I was in that exact same mind-set LAST year (after Christmas/New Years never the less!) after I did my first round of Lighter Life. First came the excuses for the low-calorie snacking, then it just kept going and soon enough, the low-cal became the normal stuff, because the 'healthy' option just wasn't good enough anymore. Too bland, too boring, etc etc... I went back on abstinence in February for a week, then did Lite until June, which really helped to get my head 100% in the right direction.
Granted, my relationship with food has swung too far the other way now (but slowly becoming more normalised and healthy), but, maintaining on my own, I know - that to be a healthy weight, I have to change my mindset - which I have. Like you, I have to justify the food I eat. If there's no reason for it (i.e. I'm not hungry) then, why have it? Sometimes, it's ok to have something for PURE pleasure - but those moments shouldn't become a regular thing. You are 100% correct in watching your 'Skinny Cow' hot chocolate and ice cream treats, alternate the days you have them, because, I expect, you're a creature of habit... once you get used to a routine, you will follow it and grow comfortable with it. It's so easy to add things on to a comfortable daily habit. One skinny-cow hot chocolate will become a standard, and another may creep in, in the morning ... and so on.
Sounds like you had a brilliant (and vigilant!) New Years and Christmas - and, hopefully you're keeping warm in this dreary freezing weather! At times like this, I kind of wish I still had some bodyfat! LOL!