'kids in my family' vs. 'kids at my family'

[Foka]

English Grammar Tests, Elementary Level

ESL/EFL Test #50 "Elementary Grammar Questions", question 4

There are 7 kids ......... my family.

(a) to
(b) in
(c) of
(d) at

English Grammar Tests, Elementary Level

ESL/EFL Test #50 "Elementary Grammar Questions", answer 4

There are 7 kids in my family.

Correct answer: (b) in
_________________________

Why is not a preposition "AT"?

Foka

[Aydincelenk]

Hi Foka,
As far as I know, it's because "family" is not an abstract noun in a sentence like that one. Family in this sentence is a concrete noun for a group of people and the seven kids are parts of that concrete group.
"At" as a preposition is usually used before nouns with their abstract meaning.

I hope that it is clear enough.