In which sibling combination is there greater mutual love and care: (Elder brother, younger sister) or (Elder sister, youngdr brother)?

I don't think it can be tied to a relationship. It comes down to the individuals and their personalities. 

It has nothing or I'd say very little to do with any combination in that way. It depends on kind relationship that siblings are sharing.

Impossible to say. Depends on too many other factors. How far apart are they in ages? What are the family dynamics? Are their personalities compatible?

I don't think it's about who's the elder, it's about the relationship build throughout our childhood.

Elder brother younger sister. This is where the least competition is found between two siblings.  Brothers also feel protective of their sisters, especially younger ones.  This is my observation through life. 

In my opinion, it's random. 

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