I'd like to know whether you can say: There are four members in my family.
There are four people in my family.
Could both of them be correct?
I agree with you. Your solution is much more common in NA English, though both are correct. We would most often refer to 'members of my family, though we would not say "There are four members of my family."
One more question is whether we can say: I was smoking when I was a teenager as an equivalent to : I used to smoke/ I smoked
I think not. We would not say "I was smoking when I was a teenager" We would say instead, 'I smoked when I was a teenager' or 'I smoked as a teenager' or simply, 'I used to smoke'.
Interested in Language