Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
How did they come to be joint owners?
What agreement was made about repayment?
Did your grandparents leave a Will?
have you looked at the Entry on the Title Register - what does it say?
When you say "deeds"is this Registered or unregistered land?
you need to look at the Office Copy entries - available for £4 here
When you have them let me know and I can tell you what you need to look for (it will make more sense with it in front of you)
are these words
No disposition by a sole proprietor of the registered estate (except a trust corporation) under which capital money arises is to be registered unless authorised by an order of the court".
Anywhere on the Register?
In that case legally it is for your Aunt and Uncle to sell the house and to pay you a third of the Equity recovered as Executors/Administrators of your mother's estate - for you to distribute between you and your siblings.
I hope that this is of assistance - please ask if you need further details