I think YABU.
You can't force your GA to love your DD and DS equally and if she pretends, your DD will see through it. I disagree children have a 'right' to be loved equally by all relatives (other than their parents) because special bonds can't be forced. It takes years to build up this type of rapport, and sometimes people just don't click. The same way children have favourite relatives or love one aunt more than another aunt... would you try to force your children to love all relatives equally?
It's natural GA is closer to your DS, she's known him 5 years longer and spent lots of 1-1 time with him as a young child. Maybe now she doesn't have the time, energy or inclination to do the same with DD. Maybe she finds DD irritating or hard work, or feels DD comes between her and DS if she has to take them both out. Maybe she feels DS needs her more than DD, or understands him better, or sees more of herself in him.
I have a close bond with my cousin's 8-year-old DD1, I've known her since she was born, used to babysit a lot and take her out. When she was tiny I was a student so had lots of free time to do fun things with her. Now I have a much busier life, am pregnant, tired, have other commitments... but I try to take DD1 for special outings when I can. However I don't take her younger sister DD2 (age 3) and have no intention to start doing so any time soon. I haven't bonded with her in same way. She is at an age when she needs lots of attention and entertainment; if I'm honest I find her quite irritating and exhausting on a 1-1. She has a very different personality to DD1 (DD1 is quiet and thoughtful, DD2 is loud and likes to show-off). If my cousin started expecting me to take them both out I'd think it quite rude. I'm not providing free childcare, I'm spending quality 1-1 time with a child I've cared for since birth, who will always be special to me. (Obviously I always make a fuss of both girls when I'm at their house, I buy them both presents at Xmas and birthdays, I sometimes join them on family days out and help equally with both children... but 1-1 outings are different. I don't feel I can safely manage both of them at once, and I want to spend my (limited) time with DD1.) Also, when DD2 was born DD1 became very jealous and felt pushed-out, so I felt she needed me more than ever. She still gets possessive over me around her sister... I'm happy to continue being her 'special auntie' and my cousin seems very happy with this too.
If you try to force your GA to take your DD out as well as DS, GA is likely to withdraw from DS and DS will lose out on a very special, precious relationship. I think it's lovely she gives up her time for DS and nurtures this relationship. He will benefit greatly from it.
Rather than make a big deal out of it or make demands on GA, I suggest gently explaining to your DD that GA has known DS much longer and they need time alone together. In time your DD may develop a special bond with a different relative or family friend, or she may develop her own special bond with GA as she gets older.