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Normally, spouses would hold any jointly owned property as what is called "joint tenants", which means that upon the first spouse dying, their share in the property automatically passes to the surviving spouse. Therefore, if your parents held the proeprty as Joint tenants, your Mother's share would have passed automatically to your Father upon her death, meaning that the property then became his and his alone.
You would need to instruct a local Solicitor to ascertain if the above was the case, but my feeling is that your parents probably did hold the property as joint tenants. If they did not, then they would have held it as "tenants in common" which means they both hold a separate 50% share in the property, which share they can leave to whoever they like in their Will, or if they did not leave a Will, their share would pass to their next of kin (in your Mother's case, her next of kin would have been your Father). Hence, it is more than likely that your Mother's share would have passed to your Father in this scenario also, unless she had made a Will leaving her share to you children for example. If so, then her half share would have legally belonged to you children upon her death.
I am guessing that especially as your sister is a Solicitor, that the whole of the property belonged to your Father at the time he transferred it.
On the basis that your Father owned the whole of th eproperty following your Mother's death, the property was his to do with as he so wished. He was therefore entitled to transfer it to whoever he wished, whether that be a relative or indeed he could have sold it and passed all the money to Charity, if indeed he had wanted to.
Whenever a party transfers a property, they must be advised of any pitfalls by their own Solicitor, who is under a duty to advise the client of the implications of such a Transfer. Transfers to relatives can of course cause family upset and for this reason, we Solicitors have to make sure our client fully uinderstands the nature of what he is doing, and also that no third party has exerted undue pressure on him to execute the Transfer. Your sister, being a Solicitor, would no doubt have made sure that your Father received independent legal advice before proceeding with signing the Transfer.
The only grounds you therefore have to dispute the Transfer is if you have evidence that your sister exerted undue influence on your Father to transfer the property to her son. This is going to be quite hard for you to prove, bearing in mind no doubt a Solicitor dealt with the Transfer on behalf of your Father.
I hope this assists you and sets out the legal position you find yourself in.