I use both your input and the words brother and sister. For instance, she'll refer to you as her brother, but if she's use the word bro, or whatever you want, in place of your name on some occasions. If she's shouting at you, she'll use the variable, if she's saying 'my brother is odd' then it doesn't because I think it's better that way.
Are you trying to make her your cousin or daughter? If so, that doesn't really work. I could add that functionality if people want, I just didn't think anyone would due to the MC being the samea ge as her and cousin just being a worse version of a sister IMO. I could replace all uses of 'brother' with a variable that you could change to cousin. The issue would be that it wouldn't capitalise at the start of sentences.