I parsed it like this:
もう - already
10歳 - 10 years old
か… yes?
つないだ - plain past of 繋ぐ = “connected”
その手 - it’s either “that hand” or “that kind of, that way”
は - topic marker
双子 - twins
の - possessive particle
姉弟 - siblings (this wasn’t in Jisho and is normally written as 兄弟 - perhaps these kanji imply that his siblings are a younger brother and a younger sister?)
より - more than
しっかり - tightly
していて - is doing
He’s already ten years old… that connected hand is doing more tightly than his (twin) siblings
or - He’s already ten years old… his grip is tighter than the twins.




