There is significant philosophical reflection on the parent-child relationship and the associated duties. But the sibling relationship is, as far as I know, largely neglected. But it's very interesting. Few have a choice whether to be a sibling. Many of us are already siblings from the first moment of our existence, and those who became siblings later were rarely consulted by their parents on whether they wanted to do so. Most of us who are siblings became siblings in childhood, and I suppose we could say that our parents had the authority to make us into siblings at that time. But one can become a sibling in adulthood, too.
Parenthood can be at least in part (and only in part, I've argued) ceded to another by adoption. But while Western culture historically does have siblingmaking (adelphopoiesis) rituals, these are merely the creation of a new sibling relationship rather than the transfer of the relationship. (One might think that adoption transfers the sibling relationship. I am not sure about that. But in any case, adoption is typically a decision by the parents.)
So not only do we typically become siblings with no initial choice, but we have no choice whether to remain siblings. This is made easier by the fact that in the ordinary course of things, duties to one's siblings are less onerous than duties to one's children. But that is only in the ordinary course of things. A stepmotherly nature--or a Providence that cares more about character than comfort--can throw us into circumstances where our duties to siblings are extremely onerous. (This also illustrates a comment from Mark Murphy that we should not expect moral burdens to be equally distributed.)
But what are the duties we have to our siblings? How do they change with the age of the siblings and other differences in circumstances? How are these duties spread among a multiplicity of siblings if there more than two?