How come my older niece and nephew didn’t acknowledge my grandpa’s death?

When my brother (half brother) told his son and daughter (my nephew and niece) about the passing of my grandfather, they got silent and said NOTHING. They didn’t say I’m sorry or anything. They just didn’t say anything and said “oh okay”. They should somewhat understand because their grandfather, who was my dad, passed away almost two years ago. My nephew is 15 and my niece had just turned 13 last November.
They didn’t even once say anything to me or avoid me. I miss my grandfather. 🙏🏾😭😭💔