Reading is a skill, the more you work at it the better you get. I think I'm a better overall piano playr than my late sister was, but she by far blew me away sight reading. Well, most people blow me away with sight reading...I can only limp along because of my composing activities, which keep rhythmic notation and the notes fairly fresh in my mind.
But it's really a choice of mine. I'd rather spend time playing the piano and improvising, and eventually composing a piece, THEN start the process of writing it down. My sister was the opposite - she never cared about composing, but loved to play. Hence she work a lot harder than I ever did to sight read.
And as others have said, it's not a skill that just appears. It takes a lot of hard work to get to a high level. And I agree - it's not good to compare yourself to a superior player and get down on yourself. Try to use your mom's skill as a motivation to get better.
If I let all the piano players I've seen and heard that are better than me get me down I would have given up, oh, by the time I was 8 maybe.