I'm aware this is a privileged problem to have, but nonetheless it's getting to me.
My Nan, who is in her nineties, has dominated and manipulated my usually assertive and strong DM all her life. She's uses money to control her, even though my parents don't need the money. If you don't take what she gives, she makes her life miserable with constant phone calls and guilt trips. She has a shopping addiction and will tell you the sky is green if it suits her. My DM is still doing her hair every week even though she's losing her sight 
Calling her out usually leads to trouble for my mum so I stopped bothering a few years ago.
I love my Nan because she's my Nan and I feel I should but I don't like her behaviour, or the way she makes my DM feel.
I've distanced myself from her a lot in the last few years but she's trying to reel me back in. She wants to buy a rocking horse for DD's first birthday in a few months, which on the surface is a nice thought but has snowballed into something different. She's rejected the small, age-appopriate £70 one we picked out as she wants to spend £600+.
She's admitted this is because she wants to buy her something no-one else will or can buy and it just feels like she's trying to use DD as another status symbol to show off about. What makes it even weirder is she says it's her millionaire partner's money and he is insisting on it 
(we've never met him in 10+ years, and no idea if he really exists as she's a pathological liar). I just feel like we're all being used as pawns in some weird game I don't know the rules to. Any advice on how to extricate us from this without causing a load of trouble for my DM would be really appreciated!