Tee2072, my ex-SIL also had major 'issues' with my mother - sure Mum can be vaguely irritating, as can a lot of people in their own way, but she had got on fine with her before the kids were born, and Mum was incredibly careful not to interfere.
When Mum had breast cancer and was staying with me (which at the time was reasonably near my brother's) for a long weekend, we met up with them and the kids and SIL clung to the baby (probably around 9 months) the whole time, insisting 'he's tired, he needs me' - not a look in for anyone else; though she couldn't control the older one to quite the same extent. Mum couldn't buy them anything, as clothes would not be worn, toys would be put away, etc.... very unnecessary, really.
Heard my youngest brother, who is normally as laid back as they come, saying 'if anything happens to Mum and that woman has stopped her seeing her grandchildren, I will kill her.' - and sounding as though he meant it (they were the only grandchildren at the time).
It really isn't always the MIL.
(My MIL is lovely BTW)