While I respect that many have different training views, I cannot stress enough that I consider alpha rolls and any kind of forced, submissive positions to be absolutely abhorrent–there is plenty of evidence that they are damaging to dogs and can actually create a dangerous, fear aggressive dog. All a dog is learning by being forced into such a position in front of another dog is to be even more fear aggressive. This type of training method os advocated, unfortunately, by some of the TV trainers because they make for good TV and seem to bring quick results. And they leave a train of dogs in their wake that then end up having further problems. But this is based on totally misrepresented studies on wolves. Contemporary canine and wolf researchers have stressed again and again, that this was isolated behavior in semi domesticated wolves forced into artificially close proximity and in no way represents normal behavior. Indeed, doing rolls like this tends to be extremely aggressive behavior that creates more aggression. Dogs and wolves in the wild very rarely sink to aggression either within their packs or even between packs. I have a lot of links already posted in the training section that really debunks the mistaken theory behind this kind of approach to training and I hope that anyone who is considering using this will read more widely to understand why such an approach should be avoided.
It is not that unusual for one dog to snap at another (my Lily is a snapper, generally meaningless) –it is not necessarily a sign of aggression, and often is actually a useful and polite way in which some dogs tell another dog, often a rude dog or a more pushy dog than they care to deal with–to back off. It is not generally instigating a fight, and is rarely a dangerous precursor to anything worse. I have had plenty of dogs go through a period like this, especially if they are rescues from unknown backgrounds and may be very uneasy. All introductions, dog to dog, it should be done carefully, and if a rescue dog is uncomfortable, then I would suggest working with a CPDT qualified trainer (you can find them in your area on their website) to help work out whatever her issues are.
Without knowing the situation in which she snapped at your sister's dog, it is hard to know whether her dog is just being too pushy for your new dog, or she is uneasy around other dogs, etcm or this is just short term. Most of the time as I said, this is trivial and I'd just leave them be -- if you are having real issues then maybe talk to a good trainer for example at Dog Training Ireland for some professional advice. But really, her life has gone topsy turvy, she is being flooded with new experiences that she may find overwhelming, and she needs time to get used to all this. Snapping is defining space for an anxious or nervous dog, most of the time, and your sister's cavalier if well socialised will just give her space and they probably will be fine within the next few meetings.
Cavaliers in particular are extremely gentle and sensitive dogs, and any kind of punishment based training can absolutely break and destroy them.
I completely agree with the approach of walking dogs together to let them socialise in a neutral environment. This is what I always recommend to people whp took my rescues.