Confused about 们 in this particular sentence "你们家有几口人" I understand that 你们 is plural as in you're talking to more than 1 person; however, this question is said to one person so how come we do not say 

"你家有几口人"

Why do we add a 们 to it or am I misunderstanding something here?
"你家有几口人?" is asking the singular 'your family' (one single family as a unit)

“你们家有几口人?” is asking the plural 'you' (everyone in the family can answer this question. You as a member of the family, can answer this question on everyone's behalf)

"你的家中有几口人?" is asking the singular 'you' (how many people are in your family)

All sentences above are valid