If you have an employee (and if you are Limited then you are classed as an employee even if it is just you), no matter how many hours they do, how old they are, whether they are family or not, you need the insurance.
If you don't have employers and anything happens then you personally will be held responsible.
It is a legal (and moral in my opinion) obligation to have it!
It is not expensive though and for the peace of mind you will have, it is worth every penny.
National insurance and tax is different and your accountant will let you know what to do here. All I know is that if it a second job, they will already be using their allowances in their other wage packet and you will have to put them on full tax and NI contributions.