Using a leave in conditioner can help a ton! It would also be a good idea to dry her hair %100 and braid it before bed so that when she wakes all you need to do is unbraid and brush through.

More about the argan: It works so well, day after day, that we now only wash her hair once a week, and by the end of the week, her hair is so soft and shiny and healthy... I am jealous!

My four year old has waist length curls, and we used to have this problem as well. Detanglers just seemed to gunk her hair up even more, and we were using so much in the mornings! But last year, we started using a few drops of generic brand argan oil (like Moroccan Oil) before brushing in the mornings, and I swear to you, the tangles literally fall out while brushing. It is amazing. And, the more you use argan oil, the healthier her hair will get.

If its getting hard too brush out in the morning after sleeping before she goes to sleep brush her hair put it up in a high bun:)

I use Suave Kids Detangling Spray and "Soft and Beautiful Just for Me 2-in-1 Conditioning Detangler for my 10 year old daughter. You can always leave her hair damp and put in a high bun or braid while she sleeps, (I do that and it works for me)!

braid her hair before she goes to bed.

Use a detangling conditoner, and then rub in a small amount of anti-frizz serum into her hair. Also buy a satin pillowcase. You can pick one up for about $5 at the Bed, Bath & Beyond. Cotton pillow cases are notorious for causing hair tangles, because they snag the hair during sleep, and I bet your sister rolls around a lot. Start braiding her hair when she wakes up in the morning-as this will help keep the hair from tangling.