I think the medium size Maya wrap would work great for you and your mom.
I have the medium, and its worked great for my dh and I; the only difference in the sizes is the length of the "tail".
It was the best $40 I've spent! use it ALL the time, and dd is now 16mon. I've used it for the kangaroo carry (her favorite) when she was a babe, to secure her in the shopping cart when she was younger, hip-slinging now, also using it as a leash now too, when we are in busy, crowded public places. the back hold was the only one I didn't feel like I could do very securely.