I think a big part of it is they are children right now and we haven’t seen any pics of them as adults so it feels weird. It also has an incest like vibe especially if they start to have feelings while thinking they are siblings. I think it can still work out well though if handled properly. For example they just grow up close as siblings and the in pre or early teens they get separated for some time then while it is revealed they are not related. Then they meet again in late teens or 20s sort of resetting the relationship and starting over except now they are aware they are not related and can’t be close in the way little kids can making them aware of the differences in sex and then sort of naturally the feelings tip more toward romantic love maybe pushed along by a rival getting close to one and making the other jealous and become aware of their feelings kind of thing
At first I thought that it's better if she met 'herman' too. But the Euclid family also rotten one.. It's better she stay Dietrich and Yurik..
Due to the fact thay they are not real siblings, there might be change in the relationship in the future. I know some people doesn't like it, but I didn't see their relationship much as siblings maybe that's why I can accept the possible changes.
Let's just enjoy the update for now ε=ε=(ノ≧∇≦)ノ