Get a Baby ball python...they are small and will grow about a foot or so a year...so by the time it is full grown your sister will be older and bigger so it wont matter...i love my ball python and so does my 2 year old daughter which holds it and plays with it....besides once you get it your mom will get curious about it and more than likely adventually want to hold it and will get used to it and see that they are great pets