It’s very unlikely that, even if the SIL had written a positive report on the soccer mom’s son, he’d have gotten in.
It’s also very unlikely that, even if the soccer mom’s been nice and lovely, the SIL would have taken on the interview while clearly knowing the mom and her son. The SIL would have recused herself as all interviewers suppose to in such circumstances.
Whether the interviewer is bound by any instructions regarding recusal or not, it’d be very wise to be recused from any interviews of anyone the interviewer’s acquainted with for one simple reason: to avoid any unpleasant consequences with the interviewee should the admission result isn’t positive. With “elite” colleges with very low selectivity rate, it’s usual that most of interviewees do get rejected. With a highly sensitive process where a family’s “dreams” may be on the line, the blames can fly off in the interviewer’s direction. Most folks place too much significance of the interviewer’s role in their chances at admission.
In my 5 years of doing interviews, I’ve had one occasion where I had to recuse myself, from interviewing this girl who our family has known through sharing the same private piano teacher. I explained to the girl and her parents the reasons for the recusal, and they understood. Although she ended up not getting into the college for which she was being interviewed, my relationship with her and her family did not sour. She ended up at another excellent college, and I sent her my congratulations.