Mama! That big sister and big brother are holding hands — they're really good friends!
Literal
Mama! That sister [and-と] brother, hand-hand hold-[and-て], really close-friends-sama [agreement-ね]!
Authentic child speech. Several child-language features: お姉ちゃん/お兄ちゃん ('big sister/big brother,' warm familial forms), お手手 ('tiny hands,' a reduplicated diminutive of 手 with honorific お), すっごく ('really, so' — child emphatic form of すごく), 仲良しさん (friendship noun + さん as diminutive). The whole sentence is typical of how Japanese adults talk *to* small children or how children themselves speak.