Having never had DC of my own having been Widowed young, I met my now DH later in life and he had 2 grown daughters. We've been together around 15 years and are married for context.
I am now Nannie to his daughter's children since they were born and love it!
However, whenever my Mum talks about them to anyone in front of me she refers to them as my DH grandchildren. ie 'isn't this a lovely photo of Daves granddaughter'. I find it quite hurtful. As though I don't count as I'm not the biological grandmother.
I have pointed it out to my DM, but she's done it a few times now.
I think I'm already a bit conscious of not being seen as a 'proper Nannie' as it were, due to biology, but to me they are my GC.
Am I being overly sensitive to this?