OP - You should get good legal advice regarding IHT responsibilities etc.
I think that if the house is the total of the estate, IHT would be £150, ,000. You need to find out if you would be responsible for paying £37, 500 (1/4) of this.
Though I believe that if inheritance is tied up in property you can pay over ten years. Which would suit your brother, It would be like Rent of £3750 a year for him. Very cheap in London.
This is probably not the way it all works though, so get legal advice, then you could explain the facts to your mother who may not realise all the difficulties her Will could cause. If you were left with a bill of £37, 500 could you pay it? Or would you have to give up your share of inheritence? Your mother may not have thought of this.
I wouldn't be to harsh on your mother. A friend of mine was leaving a very similar Will, she thought she was being very fair and helping all her children according to their needs. When a solicitor pointed out possible legal problems and other friends pointed out how it could be seen from all her children's point of view, she changed her mind.
Your mother may also think she is doing the right and fair thing, but may not have looked at it from another viewpoint.
Is there perhaps a close friend or family member, who is not a beneficiary, who could have a quiet discussion with her in case she hasn't seen the full picture?