From my personal experience of having grown up with a sibling (who I wasn't close to them but am fairly close to now) and having two adult children with an 8 year age difference, who couldn't be closer, I would say the following.
Don't compare. Celebrate their positive differences and ignore the negative. Never say, "Why can't you be more like your brother/sister?" when they inevitably piss you off!
Treat them as individuals. Allow them their own interests, characters, personalities. Don't expect them to always get along.
Help them navigate their emotions. They won't always get along. Help them to regulate their emotions and resolve their conflicts but also give them space to feel. Then they don't have to accept an apology or forgive immediately.
Don't play favourites. Invariably, you will find one 'easier' than the other. Don't make that one your favourite!
Don't make the older one responsible for the younger one.
Don't force a friendship. Eg if one doesn't want to play with the other, don't make them. If the older one has a friend round, it's OK for them to not include the younger one.
Ultimately, though, it comes down to personality and compatability. Just because they're siblings doesn't eat they're automatically going to be friends or close. The important thing is that they value amd expect each other.
My brother and indidjt have a positive experience growing up and it took a lot of work on our parts to become close. We'd always be there for each other but we only see each other twice a year.
My children are in daily contact and very close despite the age difference. They're going on holiday together later this year.