To fully understand your rights and the legal implications, it's essential to consult with a qualified attorney who can review the specific details of your dad's will and the laws in your jurisdiction.
In most cases, if your dad's will stipulates that the house goes to you and your sister, then you and your sister would become the legal owners of the property upon his passing. Your mom, as the surviving spouse, might have certain rights depending on the laws in your jurisdiction. These rights are often referred to as "dower" or "curtesy" rights, which allow a surviving spouse to claim a portion of their deceased spouse's estate.
The specific laws regarding a surviving spouse's rights to the deceased spouse's property vary depending on the country and state or province within the country. In some places, the surviving spouse may be entitled to a portion of the estate, even if the deceased spouse's will states otherwise. However, this typically applies to the portion of the estate that is not specifically bequeathed in the will.
If your dad's will is legally valid and has been appropriately executed, the property's ownership should pass to you and your sister according to the terms of the will. If your mom tries to sell the house without your and your sister's consent, it could potentially be a breach of the will and might be subject to legal challenge.
To ensure that your rights are protected and that the distribution of assets is handled correctly, you should consult with an attorney experienced in estate law in your jurisdiction. They can help you understand the legal implications of the will, your mom's rights as the surviving spouse, and the proper steps to take to ensure the smooth transfer of ownership according to your dad's wishes as outlined in his will.