^^^^ But it’s a catastrophe waiting to happen once a home becomes multi-family. Because it demands that everyone follow 1 family’s rhythms & preferences, rather than each being able to follow their own, and then come together as those rhythms allow
This ^^^ really can't be reiterated strongly enough. Because gawd damn is it true. When we were living in my grandparents' house (hah... you can already see the problem by my choice of words), we did have two completely separate apartments. But... it was always my grandparents' house. It was never 'our house'. Which they never failed to point out every now and then. And while they defined the rules for all common spaces, it really always were 'their' spaces, not 'our' spaces, and they consistently tried to push those boundaries even into our own apartment.
I've spent sooooo much time temporarily living in other people's homes over the years. When visiting family/spending summers with them. During exchanges. During internships abroad. When starting school abroad. You name it. No matter how many pictures and string lights I put up in 'my' room, it never became a 'home away from home'. Because outside that room you had to follow your host's rules. And even inside your room you were limited with what you were allowed.
Add on the layer of living with family - that you aren't married to (because as Friday mentioned, it does naturally come with a different dynamic). There is a reason that multi-family housholds aren't really that common unless they function on the basis of one very dominant patriarch/matriarch who makes ALL the rules for EVERYONE. Because no matter how much you like a person and how much you're related, living together is a different beast.
And even if you cleared out space for your sis and nephew for them to make it theirs, to them it will continue feeling like they're living in someone else's house. And you yourself will continue feeling invaded. Because this scenario just isn't really natural. They also know - or at least hope - this is only temporary and if you already know something is gonna be temporary, even if you don't know how long that 'temporary' actually is going to be, it's really effing hard to make yourself feel at 'home'. Which always puts a sublimial level of tension on onself because you never can truly relax as you would be able to in your 'home'.
Until recently it's been working really well
Chances are that in addition to your anniversary season starting early they've also probably tried to put in more effort to be on good behavior themselves. Because they just continued feeling like guests, no matter how much you tried to make them feel welcomed and help them make it their home too. But one can keep that up for only so long. Add in that we're starting the 'dark season' that's really hard on a lot of people. Add in a friggin f*cking ongoing pandemic. Add in all those things your sis is struggling herself regardless (you mentioned illness. She's also probably keep having financial worries). Your nephew is coming up to teenage years if I remember correctly?
Creating space. Like Friday, I think that's really the only true solution to get along with room mates. And if it's just now starting to become a problem, even if you already gave them space that is their own? Maybe time to revisit that whole topic *with them*.
having a day of actual fun (I cinema, ice skate, cabin away) has also helped ease the tension
I also wanted to touch on this one. Because from my experience, while it very well may help ease some tension ... it can also really tremendously backfire. I know for me it more than once actually made things worse when we tried to 'do' something to take our minds of conflict. Because it felt forced. It didn't solve the problems. I felt pressured, not only to go when I maybe didn't actually feel like it but being polite, but also because I felt like I was expected to be better, afterwards, to somehow have the conflict just go poof. I also fare fairly poorly when I *know* something is only done as a proxy/result of something else and not out of its own initiative (going ice skate to ease tension instead of going ice skating because we wanted to go ice skating and nothing else, not deeper motives, no strings attached)
Just a word of caution ^^'