You could contest it, but as he has passed the 12 year mark, and you have waited so long to make a claim, I would not be too confident of your success. Are there mitigating circumstances as to why you let him remain in the home without sorting out ownership rights? Is he in poor health now or then? Did he live at home, and take care of your parents while you and your siblings moved out and acquired your own homes? Was there land involved that he and he alone is farming? Or is he just a lazy sod who couldn't be bothered acquiring the means to buy his own home, and has nothing but a free ride these past 15 years?
If you contest the case and win and & he can not afford to pay you your shares, the judge will issue an order for the house to be sold and you to get your equal share of the sale proceeds. Or he may give your brother a bigger share as he now has to acquire/rent a new home to live in. Are you and your siblings ok with tossing him out of the home he has always lived in, if the house has to be sold to give you all your share? If you contest it, all of these matters will come into play.
I would get legal advice as soon as possible if I were you. Some solicitors offer a once off free consultation where you'd be able to get some advice on your options and not be out of pocket. The Law Society could point you in the direction of a solicitor who specializes in Probate/Property/Inheiritance laws.