I always had a sibling in the same school at the same time with me. My brother was two years ahead, my sister two years behind.
At the same school with him:
He and his friends annoyingly intervened any time they found out that some guy and I'd started liking each other.
My friends thought he was cute and wanted to date him. He thought my friends were cute and wanted to date them, too. Ultimately, whomever of my friends and him did end up dating and it'd always crash and burn in some magnificent, dramatic way (usually him cheating on them) and I'd lose that friend most of the time.
We both took French to get our two years' of foreign language; we had the same teacher, were in the same fluency level, but had the class during different periods of the day.
So the perk part was:
I'd make some sort of attempt to do the class homework assignments the evening the work had been assigned, but I'd put his name on it and give it to him. He'd turn it in the next morning because he had the class during first period. The teacher always graded the homework right away and would hand it right back to use as a study guide. He'd usually get a big, fat "F" from my shitty work. Good thing was, the teacher would mark the correct answer when she graded. I would get the corrected homework, copy it with my name, and then I'd turn it in during 5th period. So, clearly I always got an "A".
The teacher would remark, "You have such a natural gift for the French language!" to me and to my brother, she'd say, "Maybe you should switch to ASL or Spanish because French is not for you." He didn't care about his grades and he didn't even want to bother to attempt the homework, so this arrangement worked out for him, too.
With my sister in the same school as me, I'd get to see whenever she would sneakily borrow any of my clothes and change into them once at school without asking or if she'd lend my clothes to her friends because (obviously) we'd run into each other multiple times a day. She'd still try to get away with it, though. I also caught her smoking with her friends behind the portables a couple times. She had a bathroom pass and on her way, she caught me and my best friend sneaking to the senior lot to ditch. We never told on each other, though.
If some rumor about the other was spreading through the school grapevine, we always stood up for each other. If we overheard anyone talking shit about us, we told each other what we'd heard and who said it and we'd also call that person out and let them know they needed to stfu. And if one of us needed anything: An emergency tampon, lunch money, a pen, a ponytail elastic, ANYTHING- we had someone we could go to.