I don't have family around me. So could easily dodge that.

But it is really stressful. And the questions. They make you want to scream some times, don't they!!
Perhaps it's just a really bad time. Maybe she's had a loss or is grieving a failed cycle. Perhaps she really thought it would be a Mothers day she could celebrate. Maybe their relationship is under pressure and they don't want to feel the eyes of the family on them. Perhaps they made a pact to just have the day at home alone to be together and celebrate themselves.

I went to baby showers and first birthdays and all that. You can't cancel those. But some of them hurt more than others.

If your family is close, then your MIL should be able to understand, or at least respect, that SIL can't be there. A family get together on a different day which has no expectations or significance would be easier.