My DM doesn't have a good relationship with her Mum. I get on well with both. My DM doesn't want me to invite my DGM to my wedding, because my DGM has a tendency to get tipsy at parties (often falling over, because she's not steady on her feet at the best of times!) My DM is afraid that she won't be able to relax and enjoy the day if my DGM is there (quite possibly true), because her role will become looking after her Mum, who also won't really know anyone else there.
I'm really torn about what is best. If I don't invite my DGM, then I can't see how I could invite my other Nan, or my aunt, who I would also really like to be there. They all live quite far away. My DP's view is that we should invite who we want, and that we're not responsible for other's behaviour from that point. We'd pay for a taxi for all 3 of the long distance family members (one of my DM's objections to the invite was having to arrange my DGM's travel), but otherwise it's up to both my DM and DGM how they behave.
Yabu - if it will spoil your DM's day, there's no way you should invite her.
Yanbu - invite whoever you want to your wedding.
Any advice much appreciated!