As an older sister (although much younger than your older sister), I do that every once in a while. If my sis was arguing with our dad, I'd probably either address both of them ("guys, what's up?) or address her...now that I think about it, not sure why, but I guess I would assume it was her fault? Which isn't to say that it IS her fault. And I've told my sister to "grow up" a million times. I guess it's not the nicest thing to say, especially in certain situations, but it's not always meant in a mean way. Like I'd say it but inside I know that I might act the same way my sister was acting when I said that to her. I don't know what your relationship with you sister is, but could you call her and talk it out? Might be better than a letter.