[quote onlychildhamster]@FluffyWhiteBird that makes sense. I mean, the SIL who lives with her MIL, she is her mother's baby so she would always be supported in terms of food and clothes so the UC or any benefits she is eligible for would be a supplement.[/quote]
My point is, the state recognises that it's not like that in every family, or if eg someone was living with a random stranger for a housemate. What if MIL was abusive? What if MIL was barely scraping by herself and had literally nothing to spare and couldn't afford to fund another adult? It doesn't matter that MIL provides younger SIL with basics. MIL's generosity, that's the additional thing, not the UC. You've got it the wrong way round. The UC is recognition that SIL is an adult in a household of one, ie her, and nobody else is responsible for her. So that money is for her basics. What she actually spends it on is upto her.
The comment on older SIL not having self preservation I'd have to disagree with. Avoiding a 'sensible well paid career job' where that's so far removed from everything she is that it would possibly make her miserable, if not actually ill, shows a lot of self preservation, insight and self confidence.
Your approach is to ensure your health by ensuring you have money to chuck at any problems which occur, to know you'll never struggle for basics and to possibly afford things like private healthcare which gives you added options than just relying on state healthcare, affording holidays to relax and preserve well-being etc.
Your SIL could have a different approach, preserving her health and well-being by ensuring she doesn't end up a square peg in a round hole, shackled to an unsuitable job by a mortgage that's a millstone round her neck, trying to live a life that goes against who she fundamentally is, just because it's what society expects. It shows courage. She's taking the risk of struggling financially in order to remain true to who she is.
She's possibly recognising what would be a disastrous path for her and actively avoiding it, for now anyway. That's not feckless, it's knowing who you are and what you need and taking steps to get it, in the face of adversity and disapproval. Now the pandemic situation is easing she's taking the next step, the internship and she's found funding for it, which doesn't involve breaking UC rules by not job searching. Just because she hasn't chosen the same path as you that doesn't mean she isn't being sensible.
I wish you and your family and in-laws luck anyway. You're very different people so it makes sense you'd struggle a little to see each others point of view. If you hadn't been caught up in a global pandemic and only socialising with the in-laws, you'd likely not be feeling the odd one out, because your life choices aren't odd amongst greater society.
FWIW I've known several (dysfunctional, in the cases I've known) families where the DC have either followed in the parent's footsteps or gone totally the other way, so I don't think your DP behaviour is odd for the circumstances, nor is his siblings behaviour.