adjective (a)

• like or characteristic of or befitting a sister(adj.all)

  Example:
  Sisterly kindness.
  

  Synonym:
  sisterlike, sororal
  Antonym:
  brotherlike, brotherly, fraternal
  

  source: wordnet30
• Like a sister; becoming a sister, affectionate; as, sisterly kindness; sisterly remorse.(adjective)

  source: webster1913