Use the childhood evidence
Nicknames, family legends, terrible haircuts, gaming arguments, sporting delusions and stories your parents still repeat can all become useful card material.
Make it sound like sibling banter
Because you provide the detail and choose the brutality, the result can stay light or become the kind of savage roast only a sibling could get away with.
Approve the version that lands best
Compare the three generated versions, choose your favourite and only continue when you are happy with the final card.