How much stamp duty would we pay in this situation?

[another20]

Been left a property by a relative 50/50 with my sister worth £400k.

If I bought her 50% out and then let it out already own my own property. BTL

If I bought her out and let my children live there rent free whilst at uni. Second home.

Could I switch between the two options depending on family circumstances.

Would we also each incur an inheritance tax?

Anyone any idea ?

You might get more help on the money matters board maybe?

I'm afraid I don't know the answer

Thanks purple. - will try there!

If you buy out your relative, you would have to pay stamp duty on the portion you purchase plus the 3% 2nd property surcharge.
This shouldn't change any inheritance tax liability which should already have been incurred by you each taking over part of the property in the first place.
There is some guidance available here:
www.gov.uk/guidance/sdlt-transferring-ownership-of-land-or-property#if-you-transfer-or-divide-up-jointly-owned-property-or-land-unmarried-couples-and-other-joint-owners

I do not believe there is any difference between the two situations? What matters is that you are a 2nd home owner, not what your intention is to do with the property.