I trained as a sky fitter 15 years ago.
So when you trained the requiremwnt was for a Sky box to be permanently wired to a phone line for billing and service aspects.
Quite a bit has changed since then.
He lives in a flat above a parade of shops so there no way to fit any cable so all he can have is his internet over the phone line . I am not sure what speed he will get but i would not be optimistic. A fitter is going round next week to fit a dish for him for the tv.
Okay, so your brother's situation falls outside of the "most flats" which is why I left the contingency. There's always some exception.
Cable: Sky doesn't deliver internet via any TV cable; dish or otherwise.
Like most of us, he will get his broadband via a phone line. I do too. Unless you're with Virgin or use a mobile router then you probably do as well.
Depending on how close the fibre links get to his home then the speed could actually be fairly good. Certainly enough to stream most catch-up and subscription services.
Whether or not he could live stream some bandwidth-hungry service such as BT Sports in UHD would have to wait until he knew what consistent speed he could get. (I am not saying he necessarily wants that; just highlighting the highest requirement service to give a range.)
Don't write off his broadband speed just yet.
Just because a bill for broadband gets paid to Sky, it doesn't mean they are delivering via some Sky exclusive network. Whether you're with Talk Talk, Plusnet, BT, Sky or most other providers, the actual wiring/fibre infrastructure belongs to BT. Everyone else pays rent to use it.
The dish he is having fitted will supply live TV. The Internet connection will handle broadband and the catch-up/download services.