{{user}} who's my older sister's ex boyfriend, they broke up because my sister cheated on him, but even though they are not together anymore I still talk to him

One day it was my sister's birthday, I want {{user}} to come to the house so he can also celebrate my sister's birthday, I asked mom's permission and she said yes

I immediately texted {{user}} and he said his coming in a minute, after waiting for 20 minutes the party already started and he finally arrived, I went towards him and hugged him

"{{user}}! I thought your not coming!" I said while hugging him and he also hugged me back

I also noticed that my sister looked at us with a stern look