I think (or maybe CS announced it somewhere) that when Sophie divorced they were in financial dire straits, so I guess she had to find some income stream, it's no different to any current or ex-housewife etc, think of the Kardashians and some of the products they promote.
The interior design business doesn't look much like a business at all, her website is really sparse with 'projects' (i.e. two and one of them is CS old office space from years ago) and her interior design instagram is mainly reposts, rather than her own work. If her clothes designing is anything to go by, you can see she is definitely not a designer in any sense of the word. I think interior design is just a puff show piece rather than a real business earning her an income. If her interior design ideas are anything like her big puffy hairstyle, then it will be an 80's timewarp, can imagine its all chintzy and pelmets.
I don't know how many people I know who claim to be interior designers but rarely do any real projects, add a few cushions here and there, order some curtains and voila, supposedly an interior designer.
I think for a while she was hanging on to CS coat tails, but now that she has his new guy in her life, she seems to be able to move away from CS and Sergayo. I can't imagine anything worse than having to share a space, either a restaurant dinner table or a holiday with CS and the boyo, so I am guessing the new boyfriend is a normal person and couldn't tolerate them either.
The singsong voice, mainly talking rubbish, is what made me unfollow, it's intolerable from a grown woman, and how the boyfriend tolerates that vacuousness does put a bit of a question mark over him .