I'd like to read some manga that focuses on the relationship between an older sibling and their younger sibling(s), although I don't mind cousins or other relations / guardians. Ideally the older character is a teenager / young adult and under 10yo for the kid(s). I'd like the main focus of the manga to be that relationship, so the older and younger siblings are both main characters (not something like Aishiteruze Baby, where it's mostly focused on the teenager).

Slice of life, please! I don't mind a little drama / romance, but ideally I'd like something relaxing and happy. No shota / loli, incest, etc.

Something like:

Yotsubato!
Flat (AOGIRI Natsu)
Aka-chan to Boku
Hinatama

Thanks!

Flower Shop Boys is adorable.

Also Gakuen Babysitters.

Yoningurashi is about a whole family; cute, but both kids are young and it might not be what you're lookiing for.