My understanding is that you cannot use the same solicitors, with sale and rent back I would guess that they have not used the same Solicitors but have stated that you need to use one of their Solicitors. They will have 2 firms of Solicitors that are used to working together and know what each other wants.
Even if a Company requires you to use their recommended Solicitors (they will probably even pay their fees) always get an independent Solicitor to check out the Contracts. Seems like you may have been caught out by small print in the documents.
If your Aunt was supposed to have a 3 year tenancy agreement then they shouldn't be able to sell. You need to seek independent advice from your own Solicitor as to the contracts and what you can do?