i need help with gel nails

HAII!! so i've been practicing on my sister using gel polish. and for now i've been doing it on her natural nails cause they are veryyy long! 

i prep them (file the nail, remove the cuticles, buff it, wipe it with alcohol, and dehydrate it) and polish them (base coat, gel polish, top coat) and OFC i cure them. But for some reason the nails ALWAYS peel within a day or so!! i've noticed that they usually peel towards the free edge. I do try seal the free edge when i apply the coats of polish. however it appears that the nail polish is not sticking to it for some reason. do i have to file it and dehydrate it more??? PLZ help