I'd hate to think I ever do this but I must confess it can be there.
I had a girl joining me in year 7 with a very familiar surname. Her older brother was... interesting. He would roll up a fag and smoke it in class then baracade himself in a room to enjoy it before escaping out the window and over the fence.
Her first words to me were "I'm nothing like my brother". I didn't need telling. I had checked if she was related, I took a deep breath, I moved on and then I did as I always do; I treated the first lesson as a fresh start and a chance to get to know them all as people rather than names/relations.
I'd have been daft not to check up a few things in advance given her brother, I've been burned by not having strategies ready if required before (but at the same time vindicated. When the 8th child in a row from the same family was expelled before year 9 left the school I got an unexpected bonus from it. I was walking down the road late at night when a group of youths set out to mug me. The one leading them stopped and asked my name. I told him and he got the guys to back off saying "he is the only one who treated X like an individual and not just as my brother." Of course X was a pain in the arse and other teachers coped with him better by being ready in advance but long term... he was coming round more in my classes than theirs. If he hadn't been expelled he would have got a D for me when failing every other subject). But at the same time I never let any preparation show. It is genuinely a fresh start for all just sometimes I have a set of work ready for a safety net just in case but the pupil in question would not get sent out for being a sibling but for having behaved in a way to earn it.
The girl in question? Sweetest student ever! Hard working, lovely and a joy to teach. She made me a get well soon card and is very helpful and polite to others. She goes through the same hell as her brother but it doesn't darken her.
You can't help but worry, you can't help but be bemused but you can't let it change you or show.
I have another female student. This one is in year 8. Her brother was a gifted scientist with a real grasp of the scientific method, a healthy scepticism for newspaper science and great knowledge. I would never, ever bring that up when trying to convince her that there is a point to studying science, to reading in general, to acquiring the damnedest bit of knowledge from anything but gossip magazines, for understanding that culture did not just start 2 years ago and that Einstein, Hawkings and Dawkins have done more for the human race on their worst day ever than Justin fuckin Beiber will ever do in his life.
I don't compare identical twin sisters I teach on separate sides of the year (sometimes one straight after the other). There is no point. They are different.
My brother did have this at school but so did I. They'd bug him for not achieving as much as me and they'd bug me for not being as good at sport. When my sister finally arrived at the school she told them not to bother even trying and they didn't as she was very different from us both.
It used to happen a lot but if you did it openly in class now you would be in a lot of trouble and it is highly counter productive. If I caught a member of my staff openly comparing a student to a sibling in class I would be having a very serious chat with them.
It happens. It is sometimes useful to try things that work with one sibling with another but it is never helpful to highlight you are doing that or get all stroppy and compare one to the other if it doesn't work. It is unprofessional and it will only create a negative relationship. I will ask about older siblings. I will use the fact that I know their parents quite well because I taught X or Y for 5 years as a stick or carrot but I will not say "your brother could do this".
Every child is different and deserves to be treated as such.