This is a thiefs guild quest to clear out a house of valuables without getting caught or killing anyone. If I go in at two thirty am the woman is cooking and gives it the get out or I will call the guards line and the guy says cant I get any piece. What is she doing cooking at that time in the morning ? and why isnt the guy sleeping?. If I go in at ten am the guy is gone and the woman is sitting at the table. I enter into some dialogue then nip upstairs. She pays no attention while I stroll around nab all her stuff and stroll out. If some stranger came into my home said my brother was an honourable man then popped upstairs I would wonder whats going on and smash his face in. What I wouldn't do whip up some stew and behave as if he isnt there.

Her lack of reaction most likely means she doesn't see you. If your Sneak skill is high NPCs will literally bump into you and displace you without noticing you.

As for their behavior (cooking at 2am): I've found that if I wait outside from like 4pm until 1am the NPCs may behave like it's 9pm or 10pm. If I have to wait several hours I will usually wait from daylight until like 11pm and then wait again just 1 hour until 12am - this usually fixes this weird behavior and they'll be in bed.

I run into that a lot too when fast travelling to places. Like Buckom suggested when you get to the city you need to be at, wait an extra hour or two and all should be 'normal'

I believe that Greta simply doesn't sleep; try breaking in when she's out of the house. Addvar should be selling fish, she should be at the market, and their daughter should be playing with the other children. You could also try helping her make peace with her brother's death and then you're always welcome (assuming you pick the lock on the door).

I find it's much easier to break into houses during the day. A little more difficult to actually pick the lock, perhaps, but rarely any interference inside.