I wouldn't either. I had this issue with my older sister a few years back. But she's different to your sister. My sister and her now DH are selfish brats when it comes to presents and always have been.
They have a big house, good jobs, two cars, two lavish holidays a year, etc. They asked for money for their lavish honeymoon in one of those sickly poems on their invite. I said I was looking something really thoughtful for them and told them waht it was. She was really rude saying we don't want more crap in the house
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I'd just had it. It was the cumulative effect of everything they'd done. In the past they've always said "we'll tell you what we want" for birthdays, xmas, etc. When my sister has found out I've already bought their christmas gift, she asked to text a picture of it to her and she replied and said I don't want it and she sent me a picture of what she did want. I bought her Angel perfume once and when she opened it her face fell as she wanted Angel Innocent and not original Angel. She asked for the receipt. I once bought her Tiffany earrings, I know her tastes and the colours she likes, they were epxensive. She didn't like them and was pissed off that she couldn't return them as they were pierced earrings.
Leading up to her last birthday before the wedding, she was telephoning and texting me constantly about having changed her mind about waht she wanted. I finally bought her what the fuck she wanted and then she called me to say she'd changed her mind.
So the wedding was the last straw. She ended up getting nothing, then nothing for Christmas, nothing for birthday, nothin for next Christmas, etc. Now she gets a card. I don't get anything either but I'm not as materialisitc as her and don't care about getting stuff.
Anyway, I've written an essay. If you don't want to give cash don't. You're not obliged to do what they want.