What is the adjective of the word sister in French?

If you wanted to speak of brotherly/sisterly sentiments in French, the adjective "fraternel / fraternelle" would be used. They come from the Latin Word fratria used to speak of siblings (boys and girls alike).

Technically, a French adjective related to the Word "soeur" (sister) is the Word "sororel" which shares the same roots as "sister" or "sorority". Absolutely nobody will understand it.