good luck with the letter Frocksy!
I had similar issues with my MIL - she visited me in hospital - I had a c section- and took pics of my baby with everyone except me. - my dh, her other golden son, herself, fil etc etc even groups of them but not one with me. I mentioned it a few days later to my lovely husband and he thought I was just tired and emotional (my dad was dying with terminal cancer at the time so all in all not a good time)
I accepted I probably was, but then there were other things - little snidey comments, they wanted dp to bring the baby to see them - they only lived 10 houses away, knew I was struggling to get about because of the c section (I had an 11 lb 3 baby and it was a very traumatic delivery like yours!) and pain, and they couldnt possibly walk down to ours because it was 'a bit cold out' (they are not old or infirm btw)
anyway MIL phoned dh and said she thought I was not being very nice to them!!! (no idea about this at all - Id always thought we got on very well) dh was sooo cross, but he gave her the benefit of the doubt and invited her down to talk to me,,,she then launched into a tirade about what an awful person I was, she cried and finger pointed and shouted, whilst I sat with my 3 week old daughter on my lap, in shock.
then she flounced off. FIL phoned and said poor MIL shes so upset and dh (who is wonderful) said yes, and so is my wife and so am I. mum behaved very badly etc etc - and we have no idea why!
I left it a few days and then emailed her and said I am willing to let it all go but I think you owe me an apology for shouting at me in my home. she wrote back that we should respect them because they are dh's parents, she didnt commit any 'crime' by doing what she did etc....and we havent seen them since.
looking back I can see this boiling up when I was pregnant, we were looking at new houses - bigger and closer to my work and she didnt like that at all (wanted us nearby where she could control things) she also wanted to look after the baby but we had already decided that DH would be a sahd - which he couldnt wait for - she said it was a 'stupid idea' (actually its turned out very well - he is wonderful!) I think she was jealous we spent a lot of time with my dad (who died shortly after) too bad for her - we knew time was very limitied.
she was always a very controlling woman, with one very definite golden child (her other son) when we told everyone we were expecting she made the biggest fuss over how golden child was going to be the best uncle (never mind what a good father DH would be etc) our daughter was the first grandchild and I think she didnt like the fact the golden son was not in the limelight.
anyway - I have cried many tears over them and being treated how I was, but now I simply think that they are not nice people and my little girl is better off without them in her life. my DP is adamant he does not wish to speak to them, they dont know where we live (we did move) so its unlikely we will be seeing them any time soon..
wow - sorry I waffled on! anyway - maybe your MIL will be more receptive. I really hope so!!