Can I make my sister the beneficiary of my life insurance instead of my mother
Do you have a better relationship with your sister than with your mother?
No, she is my only sister and she's 19 years old. I can describe our relationship as "stabby", we don't get along, we do not speak anymore
Even if you don't get along with your mother, you shouldn't cut her out of your life insurance benefits.  You might get along with your sister more, but family is important and you will regret it later when your mother stops speaking to you.
What if my mother dies first though? What then?
If you are worried about her dying first you can always change the beneficiary if she dies. Unless you're married you should consider making your mom the beneficiary, or at least include them both. That way you know your mom will be taken care of if something happens to you.