What a coincidence.Lady= The chick who made Dante realize "what's important."
Trish= The chick who sacrificed her life for Dante.
Lucia= ehhh.... she didn't really do much.
Anyways, whether you don't acknowledge them or not, they are an important part of the series (well, all except for Lucia). Its always best to give credit where credit is due (though you can ignore Lucia).
Anyways, I don't think a family tree is possible outside of Sparda's main family. Nero would just happen to be a random relative.
Of the three women you've mentioned, I consider Lucia the less important.