Try not to let this upset you. You find consolation in visiting your daughter's grave, but does the same apply to your living daughter?
I wondered when I read your post if she has gone with you for your sake, not because she needs or wants to visit her sister's grave.
She is highly unlikely to have forgotten her sister, you know, but she may well remember her better everywhere else than at the cemetary.
My husband never visited his mother's grave, but that did not mean he forgot her, or did not miss her. Her photo is downstairs in our hall as I write this. It was practically the first thing my husband unpacked when we moved here.
I take fresh flowers to his grave once a week, and have done so, since the week after his funeral. I have no idea if I will go on visiting his grave so regularly - it is now 18 months since he died.
I don't visit my parents' sister's and aunt's graves, as they are all at the other side of the country, but I certainly have not forgotten any of them.
Please allow your daughter to remember her sister in her way, and do not let yourself be upset by this.
How we react to a bereavement and how we live on past it, is so individual that it is pointless to worry about someone else tackling this challenge differently.