The characters have shared history or memories.
Here is a guide to understanding, correcting, and using this phrase. shinseki no ko kara to o tomari de kara
If you're interested in learning more, I can try to provide more information on the series or find a relevant article for you! The characters have shared history or memories
If you heard this phrase spoken, it might have been: "Shinseki no ko kara to, o-tomari de kara?" – “From a relative’s child, and from staying over?” — which is still awkward. shinseki no ko kara to o tomari de kara