Things keep breaking in our rented property. Is the landlord's father capable of doing this?

It is 100% legal for the landlord to fix things themselves, or send family. There is no law anywhere dictating to the property owner who they do or do not hire. The government has a lot of control, but it is not that bad yet. I sincerely doubt that as long as the Constitution is in effect there will be any such law against home owners repairing their own property.