What a bunch of nonsense! Paul himself said that we don’t mourn like those who have no hope (1 Thess 4:13), but he did not say that we don’t mourn. We are humans. I would agree to tell someone his/her loved one is in heaven, with Jesus. But to walk up to a 10 year old and tell her it is good that her sister died... well, that’s wrong! I don’t doubt the sister meant well, but the way she said it...it was like telling me I was wrong to mourn for my sister. Were my parents wrong also? I don’t remember them throwing a party! My most vivid memories are trying to watch tv, hearing the doorbell and, knowing it was someone else coming to express condolences, my mom would start crying again. While we are at it, why express condolences at all? Should we do like the sister and just tell someone that we’ve heard the great news?
“it was like telling me I was wrong to mourn for my sister.”
She said nothing of the sort. She simply said, that your sister was in a better place.
You find it offensive because you didn’t understand what she was saying, and didn’t understand that it is what the Catholic church teaches. So either you didn’t get taught it (despite, if we accept your claim that you were Catholic), or that you didn’t understand this when it was taught.