I [27F] feel unreasonably upset about potential inheritance from mom [60sF]

I'm 27F, mom is 60sF. Also in the picture: Sister, 34F.
Tl; dr: I feel my mom will not divide her inheritance as equally as she claims she will, with my sister ending up with almost 2/3 of all properties. Not sure how to get it out of my head.
My parents owned three real estate properties. The family home in the suburbs, another house away from the city, and a studio apartment on the other side of town. All three properties have always been occupied by various family members per agreements.
It has been drilled into us since we were kids that my sister will get the family home after parents pass, I get the apartment, and the house away from the city will be split 50-50. Studio apartment deed has me and my mom on it; family home had only my dad on the deed (purchased before marriage). I never questioned the arrangement.
Dad passed some years ago and sister inherited the house as we found out he put her as sole heir to the property. Will dated a year before I was born. Whatever, it's fine. Sister is gracious enough to let mom and I continue living there. Studio apartment is currently occupied by our relative who went through a hard time.
Recently mom began talking how she'd like to split properties between my sister and I after she passes. She mentioned to me she wanted to give my sister half of all her estates, including my mom's half of the studio apartment. At least she would have half a claim to it, from what I understand.
I haven't told my mom but I feel hurt by this. My sister already has a guaranteed 2 bdrm 2 ba suburban home we are moving out of to make room for her future family, she will be getting half of the 'country' house, AND a half of the studio apartment? While I get half a studio apartment and half of the country house? I just. I feel hurt. And I don't know what to do about it.
I am not entitled to mom's estates. How can I get over what I perceive to be an unequal split?


Source.