There's almost 6 years between me and my only sibling (I'm the youngest.)
Looking back, as a parent myself, I don't think my parents invested enough time into making sure that my sister didn't feel pushed out/jealous and/or resentful. Consequently, she felt all three big time, until she reached her late teens/early 20s.
I don't remembering them nurturing our relationship at all, and they weren't ones for spending time with us.
My mum also put too much responsibility on my sister, I think. Looking back, as an adult and parent, I think if I'd have been my Dsis, I'd have resented me too.
For example, she was expected to take me out for the day, when she was about 14, even younger, so that my mum could have time to herself
, but then, that kind of thing was probably acceptable in the late 70s and 80s
.
If it had all been handled differently, I doubt that the resentment/jealousy would have ever been an issue, and we'd have been much closer as children.
So, it wasn't down to the age gap as such, but to how my parents handled the age gap, I think, and their whole approach to parenting, and personalities, of course.
As parents now, I think most of us are more mindful and pro-active about trying to encourage good relationships between our children, and ensuring, to the best of our abilities, that older siblings feel especially loved and special when new babies come along.
Even though it didn't work for us, I think a 5/6 year age gap could be a really positive dynamic.
My Dsis and I are close now.