Couldn't she just name them to you in her will?
No. You can name a guardian in the will, but the courts will take that as a guideline, and not follow it blindly.
For example, if there is a biological father, then he will get custody no matter who is named in the will unless he is proven to be completely unfit.
I've heard of a terrible situation where a poor woman dying of cancer was trying to provent her abusive ex-husband from getting the kids when she died. The ex-husband had nothing to do with the kids, but wanted to social security money the kids would have and to just be vindictive jerk. All the mom's relatives would be able to do was wait until the biological father was abusive to the kids and CPS to pull the kids from the house for them to get custody.
In short, kids cannot be willed. They aren't property.