Couple of things that I've observed in my family (failed to foster a strong bond) and my friends families that did.
1 - Never make them feel like they are in competition, or competing for the same emotional, financial, love, from you. That is the biggest thing that will kill sibling relationships.
2 - Achievement for 1 = achievement for all. Friends who have strong bonds view family more like this. Oh, if my sister does well, then that is a good thing for me. We are a family, we all help each other, so an achievement for one person potentially lets them help us more, if we ever need it. etc. We succeed together, or fail together etc.
3 - Play together, activities together (easy when they are young, but needs more work when they are older..)
4 - MUST foster a sense of care/protection from the older sibling to the younger one.
I think most older siblings views can be categorized into two groups:
a, This is my younger brother, he is so annoying!
b, That is my younger brother, back off! (protective role)
I think that making your older child feel protective, and caring for the young one, is the only way to balance the annoying nature of younger siblings. Young siblings are always annoying, but if the older one views themself as a protector, guide, mentor, role-model etc. e.g. "My big brother, he is always looking out for me" then the annoyance isn't a big deal.
A bit sexist, but I think parents are much better at forcing older brothers to do this, than older sisters. Probably because of generations of men needing to protect women, but I think a lot of families push their son strongly to "she is your little sis, keep an eye on her, help her, protect her etc." which then leads to the trope of the protective older brother.
Not all (i've known some very nurturing/caring big sisters) but many big sisters fall into the 'Ugh, that is my stupid annoying brother' category instead. I don't think this is their fault at all. I think its mainly because their parents failed to push them into a caring role at a young age, so the only emotion they have is generated by their interactions. Younger children are generally always annoying to older children, so of course their relationship gets defined by that annoyance, and not by a nurturing/caring/protecting type role.