Explanation:

In a family of seven persons – Sulu, Sam, Sid, Saya, Shri, Sara and Suni, How is Suni related to Sid?
Statement I: Sulu is the elder sister of Sam, who is the only son of Suni. Sara is the father of Saya, who is married to Sid.
Statement II: Sid is the brother of Shri, who is unmarried. Sara is married to Suni, who has only one son Sam. Sulu has two siblings.
Checking statement I:
Following images can be drawn from the hints given in statement I.

But we cannot determine the relation between Sid and Suni, because we don't know whether Suni is married to Sara or not.
Hence data in statement I alone is not sufficient to answer the question.
Checking statement II:
Following images can be prepared from the hints given in statement II.

But still, we cannot determine the relation between Sid and Suni.
Hence data in statement II alone is not sufficient to answer the question.
Checking statements I and II:
As Sara is married to Suni and father of Saya, who is Sid's wife. Thus Suni must be Saya's mother and thus mother-in-law of Sid.

Clearly, Suni is the mother-in-law of Sid.
Hence data in statement I and II together is necessary to answer the question.