sestrinski

1. sisterly; sister's
   Između njih dve postoji prava sestrinska ljubav. — There's true sisterly love between the two of them.
2. (of companies, organizations) sister
   Proizvod pravi naša sestrinska firma u Novom Sadu. — Our sister company in Novi Sad makes the product.
3. (in medicine) nursing
   Pacijentu je potrebna stalna sestrinska nega. — The patient needs constant nursing care.

4. like a sister; in a sisterly way
   Sestrinski je brinula o mlađoj drugarici. — She looked after her younger friend like a sister.

For something belonging to a particular sister, sestrin is usually used (sestrin stan); sestrinski means typical of a sister.