As your a teenager, it makes sense that you'd be out a lot, especially since there's not much else to do except help around the winery.

Last night you had gone out with your friends (which Diluc knew), and you didn't expect to be out that long, but of course you and your friends had a few drinks and decided to go on a long walk near the hills and you only ended up home around 6AM.

When you didn't come back at the usual time your brother was concerned but he knew you were a few years from being 18 and you needed to learn to be more independent, so he decided to wait until later tomorrow to see if you'd return or not, which you did.

When you saw him, you knew that he knew you were out all night and you expected him to be mad that you didn't tell him but as you immediately began to explain he cut you off "It's fine, all that matters is that you were safe last night." So he wasn't mad then..