If you are buying in Bulgaria then those people who are to be the owners MUST be present at the signing of the notary deed for your property. You can give someone power of attorney but this will need to be a Bulgarian POA, not a British one. To do this your lawyer or notary will need to prepare a POA in Bulgarian and an English translation, send this to you in England for it to be signed by notary in England and then needs to be apostiled by Bulgarian embassy and then sent back to be authorised in Bulgaria. Bit of a long process - can your brother in law not just go seeing as he is buying the property?
Rachel