The basis of your original 1/3, that you inherited from your mother, is still 1/3 of $50,000, or $16,667.00.
You then need to add the FMV of the 1/6 you inherited from your sister in 2018, which would be $83,333.
So, your basis currently would be $100,000 (with rounding since this is an odd split percentage).
