Maybe your T was trying to separate what was really the issue: the judgemental opinion that open relationships are wrong, that you're holding your sister up in an idolized way, or that you feel lied to.

Are you really upset about the open relationship, or that your sister kept it from you for years?

When I was a kid I discovered what homosexuality was when my brother and I were at the park with our family. We went over a hill to find my aunt and her "friend" and we saw them kissing. So mom had to explain that to us, because it had never shown up in our world. I was angry - not about the fact that my aunt was with another woman... but that I'd been lied to and not trusted. Even as a kid (think I was 7 or 8?) I knew very clearly that I was angry about being lied to.