1. As the person with power of attorney he presumably had the right to sell the land.
2. The problem then is what he did with the money. It should be kept separate from his own money & used for his mother, or invested on her behalf. Proper records of its use should be kept.
3. He may argue that, as half of it will eventually be his, he sees no reason why he should not use it now. This is wrong, but (provided there is & always will be sufficient money to pay for his mother's care & other needs) may be seen as a lesser problem than if he is also eating in to the half which should eventually go to his sister.
4. I don't know what could happen to him - it depends on the detail of what exactly he has done. However, you are clearly concerned about this. If you wish to do anything then you should either inform the sister's husband of what you suspect, or contact the Public Guardianship Office (which has responsibility for powers of attorney). I must say, I hope you do that because it is quite wrong for him to be allowed to go on using money which is not his, especially if it later on deprives his sister of what is rightfully hers.