My DM has always had a divide and conquer approach where DSis and I are concerned.
Like yours, she had a difficult relationship with her mother and no sisters, only brothers.
She has no female friends apart from people a generation older than her that she feels are no threat to her.
She has lots of other issues and is a hugely difficult person to be close to. She is extremely ‘toxic’ and it’s weird that so much of what you say chimes with our experiences.
Your DM doesn’t sound remotely as difficult as mine - there are loads of other issues.
DSis and I were not at all close until we reconnected as independent adults.
DF died and we had to cross the world to scatter his ashes with DM, our unsuitable boyfriends, my DS1 various bonkers relatives etc.
We had a drunken, shouty, snotty row about our childhood where, of all the things we could have said - I cried about how ugly our DPs made me feel. She cried about how they made her feel stupid.
Sometimes parents get stuck in habits that started kindly - ‘isn’t mrssapphire bright.’
‘Isn’t mrsSaph’s sis pretty’,
If your DM is otherwise lovely then do gently bring it up with her. While it’s hard to articulate without feeling petty, it’s a very public statement of value and it hurts.
YANBU