To what extent is it lawful to bear full responsibility for the loss of an investment in which the profits were supposed to be shared, without informing the original partner, knowing that the intention was to guarantee the money?

This transaction is a Mudarabah contract. Guaranteeing the capital is not permissible in it, and the intention of guarantee does not affect the validity of the contract unless it is explicitly stated in the contract. The sister is not allowed to demand her full capital back in case of loss; rather, she takes what remains. Specifying a duration for Mudarabah is a matter of scholarly disagreement. The Hanafis and Hanbalis permit setting a time limit for Mudarabah, while the more preponderant view is that setting a time limit is not permissible, which is the madhhab of the Malikis and Shafi'is.