Ediese I has your sister considered BKT?
The reason I say this is that I am using it to transition. When I first got it done my hair was bone straight and thin, however after the first wash my hair has a little volume but is soft and manageable. My plan is to progress my BKT to the point where I can wash DC and then style with roller-sets, curl-formers, bantu knots, braid outs and twist outs, all of which can be a little tricky on wet natural hair due to shrinkage. I still get shrinkage but it just feels more manageable.
Also your sister can try out the treatment on small sections of her hair at the nape, crown and front and see how it goes. The first treatment will not have such a huge effect that it would be glaring obvious and stick out.
One word of warning though is that as your sister has superfine hair like me I would advise her NOT to put the BKT on her relaxed ends or just lightly apply and do 1 or 2 passes and instead really focus the BKT on her roots.
HTH's