My kids are 6 years apart. I love it.
My 9 yo is intense in all ways, and that includes her in-the-moment love or hate for her 3 yo brother, lol. Most of the time, their relationship is very loving. Every day they play together, she reads to him, and they hug, kiss, cuddle, etc. Sometimes they share a bed (they shared a bed every night until about 6 months ago when dd moved out of the family bed. She sometimes still comes back.)
And....every day they aggravate each other and she declares that he is "EVIL!" or some other drama. I think a lot of that is just being 9, or being dd
Some things that I think have helped:
When the older sib is jealous of some percieved "special treatment" that the younger sib gets (a toddler getting extra help getting to sleep, or being expected to clean up when the younger sib is not), I tell stories about what it was like when the older sib was a little one. I tell her stories of all the ways we doted on her, all the undivided attention she had, how patient we were with her, etc. It isn't in a nasty "you aren't a baby anymore!" tone, but rather in a "remembering her babyhood with fond memories" tone. She LOVES those stories, and I do think it helps her understand why things are "fair" if not always "even".
Also, we foster a friendly attitude of "us against them"
Again, this is playful--not serious. But getting them to "team up" against the parent/parents in playful ways fosters a sense of closeness and interdependence in them.