My mum and sister both own a 50% share in a buy to let. Sister wants to sell, and mum wants to buy her out. Will mum just have to pay stamp duty on 50% of the value of the house, or on 100% of the value?

Secondly, mum is well into her 70s. Will she have trouble getting a buy to let mortgage? Assume that all the financial are in her favour, just her age potentially against her.

Sorry i dont know about stamp duty between family transfers but i think between husband and wife transfers are stamp duty free, or they were a few years ago.

Just be aware a disposal of your sisters share is taxable via cgt, theres an 11k allowance and you can offset purchase and sale costs, plus any other
capital costs.

Have you Googled it?
Can I suggest you do?

Edit, just looked at .gov site sdlt is due on transfers between husband and wife, taxed on the value of transaction not the total.

As above paid on 50% value, remember the second home/btl 3% too.