A aunt - niece series where the niece is older could be a fun variation. Say the niece's mom has her at 20, and the grandparents had kids fairly early too, so they might only be in their 40's, but then they get a late kid at about the same time, so niece and aunt would be about the same age ...
Reading this makes me laugh real hard. Since one of my aunt is almost literally 9 years younger than me.