The father and mother, other ascendants, collateral relatives of the spouses, and even strangers, may, by marriage contract, dispose of all or part of the property which they leave on the day of their death, both for the benefit of the said spouses, and for the benefit of the unborn children of their marriage, in the event that the donor survives the donee spouse.
Such a gift, although made for the benefit only of the spouses or of one of them, shall always, in the said case of the survival of the donor, be presumed to be made for the benefit of the children and descendants to be born of the marriage.