Can we conclude your meddlesome mommy broke up sister's marriage and yours is the next target?

You don't have to provide explanations and excuses. Just say you can't. Don't feel the need to justify and explain everything. Accept that your mother is stressed out and leaning on you and does not right now understand your position. Don't expect her to. Just set the boundaries you need by saying "no, I can't do that. I will do this..."

Doesn't your sister have any friends? Your mom should be sending your sister to your house to help out if she needs a distraction.

Your sister is an adult. Do what you can, and don't feel bad when you can't. Ignore your mother.

When a friend of mine was going through a divorce, we would go over there after the kids were asleep to just chat for an hour or two. We would eat guac and chips, ice cream and drink a glass of wine. Many times she was doing little things like loading the dishwasher or folding laundry. So we could connect, but she could still take care of stuff around the house. Really helped her through a tough time.

Can you set up a Girl's night where she comes over to your house after the kids are asleep?

your mother is nuts and your sister is an adult