my brothers and sisters are still younger than me but I don't know if they knew anything about crypto and wallet, but I have an idea, if it's stupid do tell me.
Your siblings since they are younger than you will respect some boundaries if you set them, it will be a different scenario if you are the younger one, but not so in this situation. Also since they are younger, in most cases you can outsmart them. Even if you write down your seed phrase on a paper, do not be foolish to labeled it "seed phrase" or any name related to cryptocurrency or wallet. You can decide to extend the words by adding some dummy words too, or removing some words too.
That strategy of course is still not the safest because you cannot guarantee that other people will not get to it.
I would still not rely on the establishment of boundaries and it would be better to secure it in another way. The safest thing would be if the brothers and sisters never know or see your seed-phrase. Of course, until the moment when you are ready to share this information with them (if it happens) and when they are ready for it.
It is best to store in a place where they will not get (outsiders included). You will have to think a little about the choice of place and method of storage. Look around at home and try to come up with your own. It can be the use of the design features of your home, interior items and something else. And of course, you should not rely on easily destructible storage media like paper.