Refer to this recent question and Skyline's answer in particular.
http://www.theanswerbank.co.uk/Business-and-Fi nance/Business/Question642976.html
Being limited provides some protection to your personal assets from creditors in the event of the worst happening. It may also have NI advantages. It has no impact on tax because the rate at which Corporation tax is deducted on a small business (20%) is exactly the same as the personal rate of income tax (basic rate).
However you can minimise some of the NI contributions by taking dividends from the business and not a salary.
Running a Ltd Co comes with obligations - including drawing up accounts and submitting these to Companies House. Of course you can pay an accountant to do this for for but there is no obligation that the accounts be drawn up by a qualified accountant. Your brother-in-law should be perfectly able to generate the accounts if he has accountancy background. I do the same for my business - I am not a chartered anything.