My husband will not speak up and stand his ground to his sisters about his mother's care. How can I get him to ask for their help?

My husbands mother lives with us, she had for 10 years. My husband is the only son and youngest. He has 3 older sisters. Over the 10 years, one of his sisters out of state would have Mom come visit about 1x a year for a few weeks; his other 2 sisters lived less than 20 minutes away and NEVER EVER just came to see her. His one sister has NEVER had Mom to her house over night. Mom's ability to do and go has diminished greatly over the last few years. She has a previous back injury that as she has gotten older has made it harder for her to ride and travel, we deal with bowel and kidney issues on the road, she is blind in one eye from childhood and now her other eye has a degenerative disease that has limited her greatly to where we have have to prepare her meals (she can fix a sandwich, get a drink for herself etc) but can not cook or operate microwave. We have always said anyone is welcome out to see her and spend time with her etc, grandkids and all, but no one ever comes. She was hospitalized in Nov for kidney stones at 76; no one offered to help us etc, we now live 2 1/2 hrs from the sisters and they never call (maybe one of them once a month), have no idea the care and needs she has and now they want her to come stay with one of the daughters for six weeks. I provide most of her care as her daughter in law. I deal with her sugar issues, now her kidney stone issues and altering other things to prevent more problems, I deal with the daily issues of normal living and now they want her so they can enjoy Easter and Mothers Day with her, knowing we wanted to take a vacation (my husband and I) in late May and part of June. Am I wrong to feel that she should not go for 6 weeks, do I have a right to say this? I feel when I tell them things they need to know about her that they think I am not suppose to speak. Am I wrong to think that they should be a little more considerate of us by talking to us instead of just deciding dates to meet THEIR plans. Sometimes Mom gets so depressed about her girls because one will call and promise each month to come get her for a week and then never show. We deal with all that. Seems that each time they all get together all they do is complain about things she says and does, but they don't seem to consider how she feels about being 'abandoned' by them. What can I do? What should I do? My husband will not speak up and stand ground to his sisters, he is way to passive compared to them. Please some good thoughts on this one would help!!