The good news is that as you used to have a great relationship, and your sister still wants to come and stay several times a year, it sounds like there's some deep emotions (like jealousy, low self-esteem or depression?) driving those comments, rather than any fundamental personality clash or nasty streak. If you're bottling it in and not reacting at the time, she may not even be aware that you're noticing the comments, and so pushing more and more, either consciously or sub-consciously, to try and elicit a reaction or a conversation?
Coming from two generations of all-female siblings, I don't know if either of these scenarios is similar to yours, and of any help?
1. My mum had a decent relationship with her two sisters until we were young. The other two then got very close and they started going away as a foursome with their husbands without inviting her, and one sister started making snide comments about random stuff. It wasn't until I was a parent myself that I thought of asking my mum whether she'd ever considered that her sisters might be jealous / struggling with the fact that she had kids (neither of them were able to). My mum has low self-esteem and the fact someone would envy her had never crossed her mind. As this hadn't occurred to her, it also probably meant she wasn't always tactful / understanding of the emotions they were going through, driving even more of a wedge.
2. One of my sisters tends to make fairly frequent mini-digs at my parenting / relationship choices. It doesn't bother me as much as it does my husband, as we still have a good relationship aside from that (which he finds weird). In her case, she tends to make these comments when she feels threatened in parenting choices, 'her' area of expertise - she married and had kids young, whereas my other sister and I had careers and travelled etc first. I've never really had a discussion about this with her, as I can see what's driving it and it’s a minor irritant rather than a big thing, but I do sometimes have a 'break' for a few weeks if something's particularly wound me up...
Christmas can be a highly charged time of year anyway, even more so for someone struggling with issues. Could you maybe arrange for her to visit sooner and try to talk it out in a non-judgemental way? Also you don’t mention whether you ever visit her – could she be feeling that she has to slot into your ‘happy family’ life, rather than it being just the two of you at hers (if she lives alone)?? Either way, it’s really worth trying to understand what’s going on and salvage the relationship before it’s too late. Good luck.