This could be a long one. I'll try to make it as brief as possible but sorry if I miss any important details in process.
My brother and SIL have been married about 8 years and have a 1 year old. SIL has a physical disability which impacts her mobility, but she is very independent.
When they met, my brother was living with my mum. He was probably a bit of a mummy's boy. SIL-to-be lived a long way away (they met online).
My mum has always been a very practical kind of person who calls a spade a spade. When DB and SIL first got together my mum expressed to me she was worried that my brother was taking on a lot - travelling across country to see SIL every weekend and him supporting her with her disability.
Apparantly she voiced similar concerns to my brother, who repeated them to SIL and they struck a nerve.
Fast forward 8 years and it seems resentment about these comments has been steadily building. Over time mum has been given an increasingly cold shoulder by SIL. She has been given repeated excuses over years to not visit them, or SIL is away/working when mum has visited (they live at other end of country so visits are not regular).
My brother confided in me last year that SIL did not want to see my mum at all. I told him he needed to resolve this and could not keep giving excuses to my mum, especially now they had a son my mum was desperate to meet.
He has now told my mum SIL wants nothing to do with her as 'it is clear you do not like her' (no further explanation provided). My mum is heartbroken and thinks she will never she her grandson again.
Clearly there's a lot of second hand info here but who is BU? Can this be resolved?