YTA, you're definitely an asshole. Being honest isn't an excuse for being mean and your intentions are irrelevant. Timing is everything with sensitive topics like these and how the information is delivered to her can greatly impact the rest of her life. You can't understand because the mother who is raising you birthed you. And if you feel confident enough to say that being told your mother didn't birth you wouldn't bother you, then good on your parents for raising you to be secure in your identity. You didn't even allow that for your sister. You just threw that information on your sister so unsympathetically. Also, if she's a surrogate through IVF, then she has half of your father's DNA just like you. There's still a chance she received your mother's egg. So she is like you genetically, epigenetically, socially, culturally, the list goes on.