I agree with this. You need to remove the need for your SIL to do anything to her hair.
I don't know what your daughters hair type is (I think I remember that you said you were mixed?) but if it's in the 3 - 4 range, you can also braid/twist her hair a la beads, braids, and beyond (http://beadsbraidsbeyond.blogspot.co...llery.html?m=1
). This way, it'll be too much trouble for her to undo the style to wash and press it.