I remember your thread from last year and it’s time for you to put your foot down. If she gets angry, tell her she is hosting next year but you’ll only stay 2 nights. If she wants to extend, you tell her, reiterate that you said these dates and you meant it. ‘We need a break from hosting, plus we simply don’t have the money’. Is she aware of how little you have compared to her? She sounds hugely inconsiderate. It isn’t her childhood home, it’s not like she has a sentimental attachment. If you don’t want her there when you aren’t, tell her!
If you’re in the pub, you say ‘Rounds’ before you go so there’s no expectation that you pay for all the drinks.
Re the having sex in the bath, be brave and tell her it’s your house, not hers and it frankly makes you uncomfortable, could they please not do that. It’s a small 3 bed house, it’s pretty obvious and demonstrates a massive lack of consideration. Can’t they restrain themselves?!
Definitely mention col and how you only have limited funds (compared to them). Tell them the cost of last year and say you’re having a romantic UK break next year and you can’t afford to fund 4 others for 7 days. I think it’s disgusting, actually, that they’re not bringing/buying at least half of the food/booze/shopping. Ideally, discuss everything before she comes. Save up and be away next year and no, they can’t stay while you’re away!
I’m angry on your behalf, given the sheer cf of her and her partner, but I think she has attachment issues (I’m no psychologist , but saying what she was like as a child is telling). Is/was she the eldest/only child for a while?