Reposting from my comment
This question has one of the most insane assumptions I’ve ever seen on the LSAT, I’m sorry but this should of been taken off the exam. The assumption that a twin watching another twin would fall more (or completely) under the watching self category and than the watching other category would be like assuming both twins looking in mirror would have difficulty telling which one they are.
D) In the studies of Identical twins, participants who observed their twin reading overreported by a significant amount how much time they themselves spent reading in the days that followed.
This cannot weaken part of the argument without strengthening the other part. Is a twin watching a twin more like watching an other or watching oneself? you know the answer. So you plug this in, it would actually strengthen the argument.
You basically have to assume that group this answer effects for sure (the watching other group) is somehow effected less than the group that you have to make a massive assumption for, so that the argument can be weakened.
Admin note: edited title
https://classic.7sage.com/lsat_explanations/lsat-81-section-2-question-22/
ah! Well met