Is it normal that my sisters cat try's to trip people down the stairs

*human tumbling down the stairs*

EVIL CAT STRIKES AGAIN. Meowahahaha!!!

Anyone who has lived with a cat for any length of time knows that cats were sent here by some alien race to destroy all humans, insidiously. Apparently, your cat missed the lesson about covert action and is taking a more direct approach.
You should realize that cats are much more intelligent than humans (after all we feed them and care for them, but they give us little or nothing in return), and your only possible course is to sit down with your cat and have a frank discussion about coexisting in the same house.
But with 9 lives, they will surely outlive us all.

It's just playing. You need to give it more direct playtime and everyone should be smart enough to be cautious of one little cat on the stairs. The cat doesn't have the reasoning skills to plot against your family. Seriously.

Sounds like kitty wants attention!

Try playing with him more. And always look to see if he's nearby before descending the stairs.

The cat is just playing. They are mischievous creatures.

My cats make it a daily goal to try and make me break a leg but they don't mean to. They are just greeting me and winding around my legs or dart in front of me suddenly.

You can try spraying white vinegar near the stairs though, cats don't like that smell and with a bit of luck, he will avoid the staircase area entirely.

Theyr`e just being playful. they don`t realise that they might cause harm with their behaviour.

He probably just wants to play. A cat's predatory instincts can kick in over any of things. I doubt he means any harm.

Yeah I don't think its intentional. I have had cats that do this. They will smack peoples legs and run by. Than they will hide in dark areas and jump out at people. Than they will come at you or wait till you run after them and hide or fall over. It sounds like they are having fun stalking people. Cats like to stalk things. They probably see it as a big game.

Next time, don't try to get out of his way. If you show him that he's going to get himself stepped on, he'll stop.

He's aware of what he's doing, be afraid...be very afraid

All cats used to be the size of pumas and leopards and tigers and then Egyptians bred them down to little ankle-nippers and the rest of the world did their best at killing off the big ones that were left. We're still trying to kill any big ones that remain.

If a little one on the stairs is offended by this, I think it might be justified. If I was a cat, I'd be absolutely fucking furious.