You're right, you're right: Kate had absolutely no right to what was quite certainly Ruairi's room at Willow Cottage but demanded it anyway. She came back from RSA on 12th February 2019, and
"Kate wastes no time in voicing her feelings about the lack of space, the lack of furniture, the lack of her clothes and much else. Brian informs her that she can stay until Friday when Ruairi will be home. ... Kate launches into an ‘it’s not fair’ rant, demanding to know when her parents are going to buy somewhere bigger. Brian responds by telling Kate that if she thinks she can live with them she’s got another think coming, and for the first time ever, Jennifer backs him up, saying it’s time Kate stood on her own two feet." (Lowfield).
So Kate stayed "till the end of the week"; and when Ruairi got home early on Friday morning, drunk and having had a horrible evening and night, and wanted the use of his own room,
"Nursing a crashing hangover and a broken heart, Ruairi goes to his room to sleep it off, but there is little chance of that as Kate screams abuse at him. Brian comes up to see what is going on, and points out to Kate that the room was hers only until today; Ruairi has possession now. In high dudgeon Kate slams off to one of her yurts."
I was intrigued enough to go and have a listen to the episode, and when he arrived at his room and walked into it Ruairi (who clearly had no idea Kate had decided it was her room now, and said he had forgotten she was there) did indeed tell Kate he didn't want to see her in her bra and clown trousers, to which she angrily responded they were not clown trousers, they were Thai fisherman's pants and if he didn't like it he could turn the light off and get out of her room. He pointed out that it wasn't her room, and she told him she didn't care about his woes and he was to go and sleep it off on the sofa, which he refused to do, saying that it wasn't even her home and she should go and find somebody else's house to doss in. Brian then intervened, having been woken by the raised voices, and Kate became extremely shrill and vituperative about Ruairi being drunk when he was sixteen. (Goodness what a hypocrite that woman is!) She was then given her marching orders and went off to whinge to Roy about it. He arranged for Lilian to ring her and offer her a room pro tem.
Thank you for making me find and listen to that again: it was a splendid example of Kate at her entitled, whinging worst.