Half-siblings would be my guess for
Christoffer and
Linnea ...
Tintin's not that rare in Sweden, it's actually unisex and more common for girls. Probably because of actress Tintin Anderzon, originally
Anna Catharina Andersson - I guess Tintin originated as a nickname for
Catharina in her case? But for a boy it's Tintin as in the comic book character, yes.