I dont actually think it is as unfair as you think.
As I understand it your parents paid £150k for his training but he was paying this back. So once he has paid it all back he would in effect have had no money off of them to keep, just the benefit of a loan.
He is intending to pay their rent/bills. This, if continued for long enough, would mean that in effect he will still be paying them back the £150k.
They are happy with the arrangement and it was their money.
You seem to be unhappy because you won't get any inheritance. But I dont understand how this situation is relevant to that as that was never guaranteed anyway given that your parents could have done whatever they liked with their own money, inheritance isn't guaranteed, and as the plan is that he will be paying their rent/bills that suggests that they do need those expenses covered so even if he was instead giving them cash by staying in his current role they may have chosen to use it for that purpose.
I'm a bit confused as you mentioned that they had been paying the money that they were getting into an account for your kids. If it is that which is stopping then surely you wouldn't have been expecting an inheritance anyway as the money was going to them not you.
It also seems odd to me that you think it is fairer for your brother to pay back the whole 150k and in effect have £0 but your parents to pay the money theyre receiving from him into your kids accounts meaning your family in effect has a large cash sum to keep. Could your brother not then think that you're actually the one being unfairly benefited instead?
The only thing I would understand is if you were annoyed you didn't get the benefit of the same level of loan as that it what would make things equal. I'm assuming you didn't have anything similar you wanted to do career wise/to better your prospects which wouldve required this sort of loan else you'd have mentioned it in your OP?
Unlike your brother, you don't seem to want to get the benefit of a loan that you'll pay back fully but work very hard using to better yourself so that you can pay your own way. Instead you want to just be able to sit back and get money given to you for nothing.
Can you see how this could all be seen from a different perspective OP?