If you were a mum of an adult would be offended if

[alligatortoss]

Their mother in law signed their birthday card from mum?

If you had a good relationship with your child.

DP and I aren’t married but my mum still addresses her card to him as ‘to my son’... it’s a sign of acceptance and welcoming , she knows very well he’s not her son and that he has a mother.

No. I used to call my exMIL 'mum' because using her first name felt a bit disrespectful but Mrs Surname was too formal. I loved her 'like' a mother but that didn't detract at all from how much I love my actual mum

Seem to be an awful lot of people who are strangely possessive about the word mum

Guilty as charged

My mil signs her cards to me this way and I think it's lovely. Hope DC are so lucky to have such a close relationship with PIl.

It's not abnormal where I'm from for people (mostly baby boomers) to call their in-laws Dad and Mom.

I personally wouldn't feel comfortable with it though I'm comfortable referring to my husband's grandparents as Nan and Grandad.

I don't know why I'd be offended with the scenario you described although it would get confusing if both sides of the family got together.

It wouldn't bother me or my mum if my MIL called herself 'mum' but I'd think it a bit strange, my MIL does have a name.

On the other hand my DH's GM signs cards to me from 'Nan', and my GM signs cards to my DH from 'Nan (name)' - she always puts her full name in brackets afterwards though in case DH isn't sure who she is (we've only been together 12 years!)