1) I know nothing about costs of building a house - I am sure you can find a website that will give you an approximate cost per square footage.
2) This is fraught with legal difficulties. I advise in the VERY STRONGEST terms for your mother (possibly with you and your brother) to seek legal advice. There are all sorts of Trust issues here - constructive trusts, resulting trusts, proprietary estoppel. It's got "litigation" written ALL over it.