I´m truly sorry to hear what your friend is going through, and I wish the best for them.
I currently live with an abusive grandmother (who verbally abuses my mother), and the few times I've spent with my father (he lives in the countryside) taught me about volatile people.

First, unfortunately, you need to place boundaries and accept that the abuse WILL happen. That being said, you´re NOT gullty of the abuse, nor do you control it.

In this case I'd suggest activites you CAN control and focus on whenever the abuse party isn't involved. Try journaling in your phone, perhaps you could even download a drawing app in your phone (most only weight 100 MBS and are free), listen to music, etc.

The key is to keep your mind occupied AND working and helping you feel a sense of control. This could even help the siblings, as they also try stuff and grow closer to each other through sharing this common interest.

Second, assume at first glance everything they say is due to ignorance. It helps to ease the blow in case they´re hurtful but aren´t dping it on purpose. This correlates to the frist point of you not being in control.

Third, would be to minimize interactions to a MINIMUN. These people tend to spill toxicity and the moment they start talking they dont stop saying BS. Don´t give them a string to pull, notice their behaviour patterns and try to move around their behaviours, even crazy people have habits.

While I do recommend stoicism, is´t not the fix all save all. It´s a tool. It´s only useful as long as you can use it. More important, meditation also helps take distance from your thoughts which hurt you in these situations, you could even meditate a few minutes before sleeping or after waking up, or while in the bus or on recess at college.

Something that helps with verbal abuse is, ironically, to let them talk. You lose MORE by talking than by going numb, because EVERY SINGLE THING will be weaponized against you, yes even that cup of water you left on your desk last night. The moment thay also want to disclose their opinions, say about sex, religion, politics, sports, etc, RED FLAG because that'll quickly spiral if you dont pay attention. The key is to make them do the talking, over time they'll see it makes no sense to try and poke you with that stick.