Is it permissible to destroy forbidden tapes owned by a family member, especially if he has hidden the Mus'haf (copy of the Qur'an), and is it permissible to destroy a musical instrument (guitar) found in the house?

The presence of prohibited musical instruments in the house is a reprehensible act that must be changed. If you are able to destroy them with your own hand without causing greater harm, then you must do so, and you are not liable for compensation. However, if destroying them would lead to greater harm, then you must denounce it verbally through advice and guidance with wisdom and good admonition.