My grandmother wants to sell her property, which she had given in bare ownership to her only son, from whom my brother and I are descended. My father died 2 years ago. We have given our consent to this sale.

Do we also need to obtain the agreement of our mother-in-law, who has opted for usufruct in our father's estate?

Yes, your mother-in-law (your father's surviving spouse) has a successive or contingent usufruct on the bare property you inherited from your father. Therefore, in the event of the sale of the property, it is necessary to involve the potential usufructuary, in this case your mother-in-law, so that : - renounce her right, - or assign it to the future buyer.