Why does a security clearance only be done on immediate family?

[murrdawg]

I am in the process of applying for my third level security clearance level, and with being a new father and common-law partner, I've discovered I have to include my spouse's immediate family in the security clearance as well. Anyone know why this is? And what happens if one of the family members refuse to give employer info, etc.? Why is a security check only done on immediate family and not extended family as well?