If your stepfather lived in Scotland, Broxy, then he can dispose of his heritable (houses and land) property in any way he sees fit. He can give your mother and his three natural sons a quarter share, give all to your mum, or all to his sons, all to two of his sons and disinherit one of them, or he can leave it to the dog and cat home. So, unless there is something that invalidates the will, there is nothing you can do about the house.
If the will is invalid (or if no will had been made), and your mother is 'ordinarily resident' in the house, your mother would have what's called 'prior rights'. She would be entitled to the ownership of the house up to the value of 130k under Section 8 of the succession (Scotland) Act 1964. If the house was valued at 200k she would have to find 70k to compensate any other parties with an interest in the house.
Unless your "stepdad" adopted you at some stage in the past 25 years (becoming your 'dad'), you have no automatic succession rights. In any case, you would only have some kind of entitlement to his 'moveable property' (savings, shares, furniture etc). However he has no right to dispose of your mother's property - that one quarter share of the house (or it's value) is hers to do with as she pleases. And her stepchildren have no more right to that than you do towards your stepfather's property. In fact, you only have a right to your mother's quarter share if she makes a will in your favour or makes no will at all.