I was telling stories about my sister when she was 4 years old to my bf about how she was extremely stubborn. I was telling them in a funny manner. He then joked that he pities whoever ends up with my sister.

The thing is my sister had a rough patch due to some mental illnesses and is quite lonely and my bf is aware of the same. I felt angry at him for making a joke like this when he knows what all she went through.