If the girl is acceptable in terms of religious commitment and character, and you want to marry her, then what you must do now is seek your mother’s approval, because your father has died and there is no way to seek his approval now. If your mother approves, there is nothing wrong with you marrying this girl. And there is nothing wrong with your mother coming with you and entering the house, and she can offer expiation for her oath.
What your father said is not a will according to sharee‘ah, and it does not have to be carried out, because the father does not have the right to force his son to marry or not to marry a particular woman, so his instructions to that effect are not valid.
A will is not valid except in the case of some clear instructions, so that it will be clear to the executor of the will what has been bequeathed, and thus he will be able to take care of it and dispose of it as directed, such as paying off debts, distributing bequests and taking care of anyone who is not able to take care of his own affairs, such as a child, insane person or foolish person. That is because the executor only carries out these instructions with permission, therefore it is not permissible for him to act except with regard to something that is well defined and known, and owned by the one who left these instructions.
It is valid to give instructions for arranging the marriage of a girl under his care, such as his daughter.
End quote from Kashshaaf al-Qinaa‘, 4/398
However, it is not valid for a woman to give instructions regarding the marriages of her sons, or for the one who is making a will to leave instructions concerning those over whom he has no guardianship, such as adult sons who are of sound mind, or his son’s children, and so on.
End quote from Mataalib Ooli al-Nuha, 4/535. See also al-Insaaf by al-Mardaawi, 7/295
To sum up: the shar‘i will that must be carried out is not valid if it consists of instructions from the father concerning the marriage of his adult son who is of sound mind, because he has no authority to force him to do that.
Marrying (this girl) is not regarded as disobedience to him, because his right to obedience ceased with his death.
But if you think that this is more likely to please your mother and spare you a lot of trouble, and you can forget about this girl and marry someone else, that may be more likely to earn the approval of the family.
And Allah knows best.
Reproduced from Islam QA