Op in the nicest way, you need to stop this situation.
only you can do it. #
your dh cant, the grandparents cant.
If I were you I would simply give your dd a whole new routine and separate the two dc for these activities.
If you dont want to cancel them, then the situation may not be as upsetting as you think.
dc survive without classes, and I am sure there is other stuff to do.
Simply stop the classes, do it abpruplty and when asked " oh, dd wasn't that keen on it anymore" casually.
if asked about what doing now, be casual, vague, not sure. and book your stuff.
start to be busy re lifts.
stop allowing the grandma to speak in such a way in front of your daughter.
its disprectful to your dd thankfully she cant understand now, but your her parent, are you going to let this carry on when she can understand?
there is no way I would subject any child to such blatant favoritism.
you either make her aware of it - ( she may not realise) or you withdraw child from such regular contact.
You can do this op.
You dont have to make a song and dance about it - I have changed my own dc activities very frequently, some classes crap, some she doesn't like etc . Its not un common and the easiet way to start taking charge of the situation. good luck.