Thanks for your reply.
The price paid by the purchaser of his previous property and the price your brother paid for his new property will be listed as an entry on the Properyt Register of the title to both properties.
If you have a postcode or an address for the property, enter it in to the following search engine and if the property is identified you can pay to download a copy of the register for the title:-
The register will cost £4.00 for each property.
Any mortgage he has on his new property will be noted as a first legal charge on the Charges Register of the title to the property. It will not tell you how much it is for and the lender will not disclose it to anyone other than your brother unless they have authority from your brother to do so.
You would have to make a specific enquiry of the Land Registry regarding searching in the UK for any other property he owns, it's discretionary I believe so they will require a legitimate reason for the search.
Property he owns in other countries and your ability to search for them will depend on the particular arrangements/requirements/records of that country.
I hope this helps, if so please kindly click accept so that I may be rewarded for my time. It will be gratefully received and you will be free to ask follow up questions.