I have 3 dcs at 3 different schools, and it really isn't a problem. ds is in Yr11 now and I've never had a clash of Parents evenings - it would be statistically unlikely, wouldn't it ? Especially as different year groups tend to have them at different times of year.
I mean, on a purely selfish basis, I hope they keep the sibling rule at least until after next year's admissions, when dd2 will follow her sister, as opposed to us having to keep fingers crossed for distance
, but objectively there is absolutely no reason why this should be so. At Primary, yes, becuse parents have to get them there, but at secondary they get themselves there, and sleep through look after themselves on training days etc., so it's not really an issue.