I agree that this is a potentially good thesis, but I suspect you'll have an uphill battle proving it. Consider Lear's curse of Goneril:"Suspend thy purpose if ever thou didst intend / To make this creature fruitful! / Into her womb convey sterility . . ." (I.iv). Perhaps your thesis can be proven in the negative. Is it possible that the sister's lack of children - their lack of maternity - is what's at work here? Consider the contrast between their conduct and that of Cordelia, who behaves in a truly "maternal" fashion towards her poor, mad father.

Hmm. It isn't a bad thesis. It is clear and specific. However, you'll need do several things to make it work. First and most importantly, you'll need to define "material love." What does this mean?

Then, you'll need to take the other factors that lead to their destruction into account. For example, they are not nearly as good at control and manipulation as they think they are. Is that because of their "material love"—or independent of it? In either case, it contributes to their destruction. Likewise, where do things like the loyalty of others to Lear fit into this thesis?