Much sex and mostly in school setting. Having a sister is a plus
One currently in progress is Al-Canadian's series called Mike and the Energists Story has six stories, two of which happen prior to the do-over.
Some older ones I've enjoyed (in some cases there are one or more sequels - not all of those involve school):
Twice Lucky by Joe J Note that this is the first story in a 3 book series with the last one incomplete.
Retreads by Rotedrachen
The Fountain of Youth by Lazlong
Echo by Reluctant Sir
Building a Better Past by tendertouch
Granted, Echo is fairly recent. A New Past was updated today but only the first half or so relates to the Main Character going to school - in today's chapter he mentions being 35.
Much sex might be a problem. 'Building a Better Past' won't qualify, not really. Sea-Life's 'Echos' the same. Al Steiner's 'Doing it All Over Again' would. The Nighthawk's 'Once More With Feelings' maybe. rlfj's 'A Fresh Start' again is light on the sex for that tag.
rlfj's 'A Fresh Start' again is light on the sex for that tag.
But what about the 'Carl Buckman Experience'?
https://storiesonline.net/s/42289/rewind would seem to fit but I can't remember if the MC had a sister, if he did then she was the youngest in the family. The story is incomplete but it did get a long way.
You may want to look at http://do-over.wikifoundry.com/. The "inc" tag may be useful for finding sisters.
That is a great resource for do-over stories. I ran across it a while ago and have read all the stories I have any interest in. I'm a little disappointed it hasn't been updated. It looks like nothing new has been added since 2016.