The correct answer is Option B.
The plural form of a word is used when we talk of more than one thing.
In the given sentence the word 'two' signifies the fact that 'I' has more than one sister.
Hence, the correct answer is the plural form of the word 'sister'. Thus, Option B- SISTERS is the correct answer.
The remaining options are incorrect because:
Option A and C are used only when we talk of anything in the singular form.
Option D is used to show possession.
Thus, Option B is the correct answer.