My mum and MIL get on quite well most of the time. They live about an hours drive from each other. We live about 3 hours from my mum and 4 hours from MIL. MIL does not drive. Every time my mum comes to see us MIL phones mum and asks her to drive to her and pick up some boxes of stuff to bring to us. Mum is coming next week so agreed a couple of weeks ago she'd pop in this weekend for more boxes.
Mum agrees politely but then immediately rings me and complains about it. I could cope it she only phoned once but she phones time after time - the most recent episode has been going on for 2 bl**dy weeks with a phone call every night when she knows DP is working away (so Monday - Thursday) around about the children's bed time which I don't answer followed by one at 8.10pm just as I finally sit down with
and repeated calls until I DO answer. She then talks incessantly for half an hour plus about how irritating MIL is and how she does not want to do a 2 hour round trip.
I've advised her to say no but she won't. I've told her I'll talk to MIL but been told I can't possibly do that or I''ll upset MIL. I've said I'll ask DP to talk to MIL but again I will apparantly upset MIL. I've suggested we tell MIL we can pick up boxes next time we go down but my mum insists this would be impolite
The boxes are nearly always full of stuff we don't want or need.
AIBU to think she can either just say no its not possible or say yes and get on with it without driving me round the bend for the next fortnight. Last night she calmly informed me I should keep MIL under control 
On top of that she'll phone me this weekend to tell me what a lovely time she had with MIL and all this will be forgotten 
What a long post. I hadn't realised quite how irritated I felt!