I have 2 DDs close in age, one of whom would probably be considered "cool" (DD1) and the other a bit "nerdy" (DD2). I dont think either thinks it better to be one than the other (just different), and i dont think they equate "cool" with "popular" in the same way as on this thread (and indeed in life).
DD1 knows her group of friends are described as the "cool" group and the "popular" group. She gets the reference to "cool" but does not really get "popular". She says that her group of friends is no larger than any other and she is not aware of people who are not in their group who are desparate to be so, so she does not see why their group is considered more popular than any other group. When this phrase is used, she will ask "popular with who?" DD2 is well liked within her year and has a good group of friends, but they are definitely queiter personalities. i dont see any jealousy towards the equivalents of DD1 and her friends in their year or the same desire to be pushing the boundaries.
I am pretty sure that issues like alcohol, boyfriends etc will come up relatively earlier with DD1 than DD2. I think your SIL's approach would be fine with DD2 as she does not like breaking rules herself anyway and in reality she is less liekly to be invited to that kind party quite so early. However, i just cant see zero tolerance working with DD1. She is very sociable and works best when she is happy,. Tryinh to stop her goign ot parties where there might be alcohol etc will in reality mean telling her she needs ot socialise with a different group of friends, which will definitely not make her happy! With her, i think it will be a case of giving her incremental amounts of freedom and ensuring she is alert to the risks, dangers of peer pressure etc, and then reining her back in if she oversteps the mark. Not sure I would go so far as making alcohol available myslef pre-sixth form, but would not judge another parent who decided to do so in y11, as long as they checked first.
So in summary i think it is easy for your SIL to judge because she is not facing the same issues. However, her younger DCs may well be more like your DD1's personality, and it will be interesting to see if she changes her parenting style to reflect that.
One thing i dont get on this thread is the idea that the cool people at school dont do well in life. That is certainly not my experience. In my school, several of the group that would have been considered to be the cool group (some underage drinking etc) went to Oxbridge and from what i can work out from social media and the school friends I keep in contact with, many of them seem very successful (QCs, professors, doctors etc). Fair play. They were never mean girl (or boy) types (I think this also gets conflated with "cool" and "popular") and it was an acadeically slective school (state grammar) where doing well in exams was not judged as uncool, so maybe that made a difference. I think in many walks of life there are advantages in being outgoing and sociable, just as in others there are advantages in being more nerdy and studious!