dd1 was the same with dd2 when dd2 was a baby.
dd1 is 4.10 now, and dd2 is 2.4, and they do play (in a limited fashion) together.
when dd2 was small, dd1 was very careful around her - very wary, really.
as dd2 got to toddler stage (9 months+) dd1 ignored her, really - she just couldn't work her out.
nw that dd2 is properly interactive (very talkative and bossy), and can play of her own accord (can ask for activities, will settle to do something at times, etc) dd1 is a lot more interested, as dd2 is a lot more predictable.
dd1 now initiates contact with dd2, has made up a few games with her (mostly of the "chasing dd2" variety, but hey, it's interaction ) and will do or say things to get a reaction from dd2.
I think dd1 now sees dd2 as a person, rather than a "baby" (which, let's face it, aren't the most interesting things to most people other than parents!) and so she now sees that interaction with her is a good thing.