I'm doing this for a Spanish project, but I can't figure it out. Would it be 'Ella es la mejor hablar de español en la casa'? I think it would, but I'm not sure.
Good guess. But your translation says
She is the best to speak Spanish in the house.
You need a different conjugation of 'hablar' to convey your meaning. The simplest change would be:
Ella es la mejor hablante de español en la casa.
But more idiomatic, and slightly more formal, might be:
Ella es la mejor hispanohablante en la casa.
But better yet might be:
Ella es la que habla español mejor en la casa.
My reason for the final change is that it makes it clear that she is the one who best speaks Spanish in the house. Strictly speaking, your sentence (in both English and Spanish) could be interpreted as saying she is the best person (by whatever measure) who speaks Spanish. Of course, that interpretation wouldn't be the most natural one, so avoiding that ambiguity may not be important. And certainly, in an informal atmosphere, people speak that way all the time.