Butthurt for picking a side?

My M-I-L takes my 1-year LO on a weekday to babysit. Thus far, it has been really inconsistent and unreliable. I see she has her Menopausal issues and sometimes lays low when we're around. I have decided to switch up with the great-aunt (her sister) who is always happy to take LO off my hands, and with her it's been great. I have asked her to take LO for this Saturday so I can do some work. Now I hear my M-IL saying to my B-IL behind my back, she doesn't get why she can't look after her grandchild afterall she's the grandmother. I'm a little put off that she won't be upfront to me about it, I can't read minds. I am trying not to take advantage of her and her time. If she wants more time with LO she should tell to ask her first. My husband tells me it's up to me if I should approach her or just let it go and maybe ask her more. What do I do with a mute M-IL???