Out of interest, what's your source for that information?
My source is not reliable
and maybe I misspoke, 10% would be a good place to start negotiations but your right of access sounds fairly weak so where would you realistically settle, 1-3%??
I can understand why you are trying to ascertain what your rights are but realistically you need to go to a solicitor to do this. But don't assume your solicitor will have your best interests at heart either. They can offer advice but will also act based on your direction so you can easily run up some very expensive legal fees.
If you 'win' and it is deemed you have full right of residence, then you should have paid a fairly substantial CAT as a significant beneficiary. There are actuarial tables to calculate this. You haven't done this so you could have 9 years of penalties to pay as well. Not great value for a property or right you have barely used.
If it is deemed you have some partial right of access and your sister can show that you have barely used it in 9 years, then the court may decide the commercial value to you is quiet small and you could very well spend more on legal fees than what you would receive in compensation.
Or you could get nothing at all.
If we want to discuss the morality of it, I'm not sure how morally defensible it is for my sibling to profit to the tune of €600,000, when it was my late aunt's wish that the property never be sold and that it be maintained as a family home
Comments like this sound like you are put out by the fact that your sister got a big inheritance and you didn't. Your aunt's wishes are not reality so it would be wise to remember that your sister owns the property to do as she pleases, not what your aunt wished.
It's fairly simple, either be practical and ask yourself have you really any intention of using your 'right' and whether it has value to you. Or, go to your solicitor and get ready to start spending money