Why was this so hard?

I’m 16F and lost my mom on Christmas Eve and it was extremely hard to watch my sister (1) open her presents. She kept calling me mama when she saw her stuff. I opened my stuff it was amazing how she had everything I wanted but I was hard because I couldn’t hug her. My sister was crawling around the house looking for my mom and I had to tell her that mommy went to be with Jesus and she just clapped her hands (probably because in the church when they say Jesus they clap) she is still a baby so she honestly don’t know what happened and she’s just responding. She still looking for mommy and today she finally got the baby lock unlocked and she looked in her room and she wasn’t there and she just started crying. I could only hug her and kiss trying not to cry myself (which didn’t work).