Little Neptune
Chang Haruka
I think I know where you are trying to go with this, and it scares me a little.
If you look around the internet, you can find evidence for both a familial and a different type of familial relationship between the two. While it is never officially announced in the series (the only official relationships seeming to be Heero and Relena, Treize and Une, Zechs and Noin, and the siblings Relena and Zechs/Miliardo), I believe it was a note in Episode Zero (At least at the end of the Trowa chapter in the Viz translation) about the two being related.
I'd have to hear a better argument for or against before I could make an official judgement on how it would be in the RP.
Since Ep Zero is written after the fact, and the only time in the series that Trowa refers to Cath as Sis is when he's got amnesia... well there is the source of the confusion. Is Ep0 canon? Can we ever really know Nanashi's past?
It seems to me that you are going to have to make the call yourself, but if you want to debate we can do that to.
I think I remember reading it somewhere, one of the creators/writers of the series claimed that it wasn't intended anywhere that the two were related, and that he found the pairing of Trowa and Catherine to be one of his favorites. But as I stated, I think I remember seeing it, I haven't seen it since, so as the girl put it, Chang you will have to make the call yourself... should it come up.