If we're going on what
@kittymeowlondon has witnessed, then Joe and Anna have been spotted acting all coupled-up, arms around each other etc. I'm presuming Kitty knows what Joe looks like and hasn't gotten him mixed up with Freddie (they don't look all that alike IMO (Joe is way cuter), but possibly have the same build, hairstyle etc). Kitty has also said that Joe has moved into a new place and has new housemates. Now, that has never made sense to me since we know Joe owns the flat Freddie lives in. He also seems to get along well with his brother - so why would he move out? My theory is that he hasn't technically moved out, but rather he's spending so much time at Anna's that it could look like he has, in fact, moved in with her. Technically, though, he still lives at his own place, hence Anna's 'Sunday' post and the fact that it looked like she was wearing pyjamas. As for them hopping around between flats - we know that lockdowns etc. mean nothing to these people!
I also think it's possible that Joe and Anna have known each other for a while - they move in the same circles etc. They haven't necessarily just met and started dating all in the last six months. Also possible: he actually has moved in with her (although that doesn't explain Anna in PJ's at Freddie's place. Hmm...)
Alternate theory: Anna is, in fact, banging both brothers. I don't call them our funtime trio for nothing!