A couple not married yet. The girl may get a large sum of money for a down payment on a home from her grandparent.

Under New York law, inheritances and gifts are separate property, not marital property.  However, if the home appreciates over the term of the marriage, that appreciation in value may become marital property.  Also, if the woman commingles the funds, by, for example, putting the money into a joint account with her fiance, it could become marital property (this result is less likely if she uses the money as a downpayment on a home that is owned jointly.)  These concerns can be obviated by a prenuptial agreement.