I don't know why everyone is on your case. Talk to your husband. This is your house too. You've been looking after your brother who is ill. What you are doing for your MIL is lovely but not sustainable. In addition to the Sundays with your MIL, you need to schedule in Sundays with downtime so you don't burn out. She may feel sad on the Sunday of your downtime but if you can make a schedule that works for you then it will only be a periodic thing, not every weekend. It's not unreasonable for you to ask for downtime to be just with your husband and kids and not be on the go-go-go running errands and sports and everything all the time.
I would start by telling your husband you would like to schedule one Sunday as downtime, as soon as possible. Nothing is to be scheduled for this day. If a last minute emergency comes up that takes away your downtime Sunday, it needs to be rescheduled for the following weekend. Over the long term you need to be calendaring together to work in some downtime Sundays. These need to be prioritized and respected by your husband. Again, if your downtime Sundays are lost because of emergencies (which always come up with elderly and sick relatives), the calendar needs to be re-evaluated to ensure you get them back in.
You are not going to have every Sunday as downtime. Even every other Sunday or every third Sunday. But even if your schedule was that you had ONE downtime Sunday every 6 weeks, you would probably feel better as long as you could rely on it and it was respected by your husband.
Your husband going to his mother's house on some of these Sundays is fine but be wary of that being ALL of your downtime, if it means you will not have ANY time just as a family together.
Your MIL is bereaved so it's normal and healthy that you provide more emotional support to her. But you are still your husband's wife, not just a random that happens to live with him nor are you merely a lodger in his home who has no say in how it is used. It is not loving to ask you to burn out from looking after other people. "I am exhausted and this is not sustainable."
It's ok to set some boundaries.