I have 10yo dsd and 14mo dd. I don't feel as if DD is left out whatsoever.
As someone said upthread, when you have a baby/ toddler and an older child, they have such different needs that you can't compare it, really.
DSD usually goes to the local shop to get sweets when she comes, she takes herself off swimming to the pool (which is a stone's throw away) she has friends over to stay and stays at her friend's. DD can't benefit from those sort of things as she's too young. I must say that DSD is a very easy going child who loves her little sister to bits, she understands they have different needs and will require different care.
With regards to trips, we usually do wait until dsd is here as she gets more benefit from going to museums, theme parks etc
If it's something that DSD has done before e.g. farm parks, zoos, aquariums then we tend to take dd on the weekend when dsd is not here. It means we can do it as playdates with children her own age.
We don't really feel like a full family unless dsd is here too and if there are things happening at the times she's not here, we always ask if she can come e.g. weddings, parties etc
DP loves the girls equally, I don't know if I love DSD (definately not in the same way as DD) but I don't think I need to, she has a mother who loves her like a mum. I act like a parent to her, not a friend and have a parent/ child relationship so I suppose I do love her, but in a different way....