Hey, :) Just been through this dilema myself! It can be worrying when you reach out to a family member in the org, for fear of rejection or maybe scaring them off, but I guess the answer I came to that there is no protocol to follow for you personally. It sounds like you love and miss your sister very much, and of course youy will want to aknowledge her anniversary. The thing to remember in these cases that it is THEM who have been restricted from assosciating or showing any natural family affection, not you!
I was in two minds on whether to send my mum and father who are heavily into the org an anniversary card, I was worried it would be slung in the bin, or worse, sent back unopened, but I realised in the end even if these things did happen, they would still know I love them, and to me the opportunity to extend love in some way to the ones I love but are left behind in the org is not one to be missed. And at the weekend when they rung to speak to the kids, my mum actually spoke to me very briefly to express thanks for the anniversary card, which I did not expect at all, although in your case with your sisters husband being a mini, you may not get such a unexpected reply (although you never know) but at the very least your sister will know you love her and are thinking of her :)