I was thinking that while Vince might not have any compunction about putting Cathryn "out on the street", he might surely hesitate from doing the same to Siwan. And also, given that he's been living with Cathryn for some time now, he would surely have established some sort of bond with her, and be a sort of father-figure. Granted that Wyn is technically her father, but given that he's a full-time wheeler-dealer, I don't see him as a "hands-on Dad" exactly, which Vince actually is. (With Wyn just being a "baby-sitter Dad" when he absolutely has to).
On the other hand (have we run out of hands yet...?), Vince correctly guessed that "Uncle Wyn" would have to come to the rescue, and ensure that Siwan at least, would not be homeless.
BTW, when Cathryn and Wyn were still living together (I don't know if they were married, but they were definitely a couple at one time), Cathryn was trying to study for a degree part-time (presumably at Prifysgol Bangor). She was having all sorts of difficulties fitting it in with childcare, and Wyn eventually employed a "nanny", who turned out to be a chap (rather a good-looking young chap, if I remember correctly). A slightly unusual situation, and I'm sure Wyn got jealous, although I don't think there was anything actually "going on" between the "nanny" and Cathryn, but there was probably a bit of "electricity" between them. I can't remember how it ended, but eventually the nanny had to go, and Cathryn pulled out of her degree course. I found that aspect of the story disappointing - seeming to give out the message that it's impossible for a mother of a young child to continue her education.
(Although to be fair, much later, we saw young Harri go off to Durham university with a baby in tow - but she had a supportive boyfriend (sort of) in Dyfan, along with her).