Yes. Big age gap (12 years) and dumped in boarding school when the twins were born because my mother 'couldn't cope' with me and them. The were, and are, her favourites. Spoilt rotten, allowed to get away with anything as far as she was concerned and when I was home I was always secondary to them - I hated them then, I still resent them now.
Making sure all the children in a family feel loved, secure and wanted is imperative. My mother and I are not as close as we were pre-twins, it took 10 years to rebuild a relationship between us but the memory of how she betrayed my love and trust still stings.
A large part of dealing with sibling rivalry is down to the parenting, making sure that all dc get quality time and attention (so that neither feels pushed out/unwanted/abandoned). I'm sure others will be along to give more advice.
The thing is, just because you may have nixed the rivalry, doesn't mean your DC will actually like each other or get along!