Something I did by instinct before even knowing it's important and even good if you live with a partner: do the boring stuff that need to be done.
Cut the grass, sweep leaves, clean the bathroom, take the trash out, cook, buy groceries, fix stuff, pay together, support the house.
All this is necessary wherever you live, but not doing make everyone who lives there stressful. An environment with less is stress is way more easy to live.
That and having a bit of boundaries, hard to create as it depends on the parents.
Oh, and don't try to change anyone, if they want to do stupid shit, as long as it don't directly affect you, let them do it.