There are a lot of reasons someone would not give a gift. Yes, they could be cheap, miserly, or ignorant of societal tradition. They could also be unemployed, or have recently incurred unexpected debt, medical or otherwise. I have been unemployed for a year and a half now. I would hate to think that if I got invited to the wedding of someone I was very close to that I would be judged on whether or not I gave a gift or how much I spent on it. I would also hate to think I would not be able to attend if I didn’t ”cover my plate”. Attending the wedding of a loved one isn’t the same as just telling someone not to eat out if you can’t afford it. If I had been invited to a wedding, I would have most likely had to postpone any major gift until I am working again.
It’s possible that the SIL is known for doing this for all holidays and weddings or this could be a one off. I don’t know enough about the situation to judge one way or the other.
I have never been to a wedding where there were desserts offered other than the cake, so when I read the OP, I assumed someone had helped themselves to the wedding cake prior to the official cutting. I am glad I was mistaken.
No shows are a big pet peeve of mine. I find it very disrespectful and unless it was something serious like a major health event or accident, I don’t know that I would be forgiving them for a while. I feel this way regardless of whether it’s a wedding, or a holiday gathering, or a simple dinner party. My invitation isn’t just a placeholder until you get a better offer. If you RSVP you are attending, be there.