If they were loans then I suspect they will need to be paid back to the estate. If they were gifts, they will need to be dealt with under IHT rules because she died within 7 years of the gift.
Speak to a solicitor. The other siblings should know what is in the will so that needs to be taken into account plus even if he claims the money he has appropriated is a gift, as it was within 7 years of her death it will come under inheritance tax rules. At the end of the day, the guy sounds...