Thanks very much everybody.
I've just come off the phone with my sister - my sister! - and she sympathised. She also has no idea what Mum's beef was with me, although she suspects it was me siding with Dad (apparently the crime that made her storm away from the table was that he had put the jar of cranberry sauce ON the saucer rather than serving the sauce IN the saucer as she'd asked).
Had I known that then I would have snuck up with a cup of tea that evening, rather than sat in confused hurt downstairs with Dad.
So I can only think I've exacerbated things since, by avoiding her I might look like I'M angry with HER, rather than waiting for her to come round (something my sister admits is probably unlikely). And it means I've not been doing the things I usually do for her - despite what she thinks, they are significant - giving her more reason to stay mad at me, as a parasite in HER house where she is unable to row with HER own husband without being judged.
So I've offered another olive branch. With the note I left saying my sis called, I added "If you'd like me to do Grandad's supper just ask." Notes are not an unusal way of communicating in this house, even when we are getting along, so I hope it's taken as it's meant.
I do know it's hard for her too.
Amd I know she's been an angel to take me in - certainly all her friends go out of their way to make sure I know.
I just can't wait for my CRB. Then I can get another job and preferably move out. Even a tiny bedsit would be better than this. Especially if I can get one on our estate, then I'll be near enough to help with Grandad, near enough to pop over and use the computer at wekends
and near enough to build bridges back up. Apart from the HUGE rows every now and then, we've done pretty damned well for two women who have emotional and mental issues living together.