A side effect of being on the internet is that you have a chance to be shipped with anything. ANYTHING . Especially if the subjects are anime girls. And Japan have been enjoying some other twincest pairings like, Hina and Sayo. If I remember correctly, HinaSayo was either shipped or accepted by almost all of JP bros while the international bros were divided because it’s incest.
So, if the girls are sisters irl, I can see why they’d be uncomfortable. But being twin sisters on the internet, they are basically saying “we have a link since our birth” which could be a huge fuel and motivation. They can ask to stop, they can ask to tone the shipping down, but it will never completely go away.
I was concerned about Advent as soon as I knew about this “twin sisters. One channel.” thing. Don’t get me wrong, it is very unique and will be huge but the shipper can get a little out of hands and may make the girls uncomfortable.
TLDR: COVER knew what they were doing and the twins are going to get shipped. They might be uncomfortable. Don’t bother the real persons while they are streaming.
Everyone can do what they want as long as they don’t bother other people.