"The sibling buying out the other is a FTB so I thought they might be exempt from stamp duty? If the value is less than the CAT threshold are they tax-exempt or are there any other applicable taxes?"
From Revenue website:
Can I avail of first time buyer relief if I previously received a gift of a house?
The relief can be claimed where the gift of the house was received prior to 22 June 2000 (or prior to 27 June 2000 in the case of part of a house). A gift received after the above date(s) is regarded as a prior purchase and would preclude a person from claiming the relief.
I honestly do not know how Revenue would treat this situation. They may regard the FTB was an FTB as it is part of the same transaction or they may say FTB used up FTB status on inheriting their share and therefore lose it for the subsequent purchase. Specific technical advice required from a good Tax expert.
"Am I understanding you correctly that if they buy out the other, then that is considered as buying a property and consequently they lose FTB status?"
After the situation ( whatever about during it) the FTB has definitely lost the FTB status.
"Would it be necessary to use a solicitor for conveyancing or is inheritance (and the buyout) simpler than normal house purchase?"
You will need a solicitor. Property transactions like this are too complex to attempt on your own. You could try it but as Vanilla recently posted:
If there are no mortgages involved then you can sell and transfer to your sister without a solicitor. You just need to draft the contracts, answer objections and requisitions on title, make sure the title you offer to your sister is in order, as is the planning and building regulations, ensure all taxable events are covered by way of clearance or payment of the relevant tax, draft the closing documents, the transfer deed and give them to your sister. Then she needs to obtain searches, make sure they are clear, stamp and register the deed. Oh and it had better all be done right cause if she ever needs to mortgage the property or resell it chances are the next purchaser or their bank will be making sure all the title and documents are in order other wise they will not buy or give a mortgage. So if you think you can do it there should be no problem."
mf