I'm worried about my sister...Is it serious or am i just overreacting??

My older sis who is 17 came to me at night and asked me if she died will I cry? I said of course I will!!!!! she said if she died she doesn't want me to get sad she told me to do her a favor and it's to not get sad if she died and to not cry then she got out of my room sad when she got out I felt like something is wrong especially she said it with a serious face I felt like my heart is burning so I ran out the room and jumped on her and said : plz don't die!!' I will do ur chores instead of u!!! I won't get mad if u used my stuff without permission!! I'm sorry for being mean to you sometimes !!! I started to cry literally she was like whaaa?! What are you talking about ?! I said I dunno just don't die she laughed and hugged me 🙃 I know that it kinda sounds funny but to be honest I feel worried I mean why did she say such a thing with a serious look and now? Or I'm just overreacting?