Marianne is Haruto's older half-sister, so big sister is correct.
Is she? Checking over the beginning chapters, there was no mention of siblings or what number child Haruto was when he was born. I could have sworn it was said that Haruto was the first, but that didn't happen and was just me misremembering and misunderstanding the King and Queen's excitement of his birth.
However, chap 12.1 Haruto refers to her as "onee-chan", which is typically used for a younger sister. And up to that point and a little further (I didn't go through every chapter in this short time), there is still no mention on whether they 2 siblings are older or younger then Haruto, and I don't recall there being a chapter later on discussing that either, though I will continue to check.
Hence why I'm asking if Haruto did say "big sis" (onee
-san), in this chapter. It would probably be quicker if I just looked at the raws myself.