I appreciate that the adults aren't shaming ostracising the kids for their very human feelings / explorations / connections.
The mom/aunt here is doing the responsible thing and trying to firmly establish the familial relationship while also accepting Nozomu as one them. She's not rejecting him or treating him like a monster but is trying to guide them to a healthy sibling-like relationship.
That doesn't mean I'm not a shipper on deck here - I think Sou and Nozomu have got a very cute and innocent relationship that could blossom into something more and real. I think the parents could be major obstacles for that if the story fully commits there. But as is this is what a responsible parent who notices what's going on would and should do in the situation. Even though cousin relationships are legal, and even if they end up having a healthy and lasting romance in the future, they've got fused family boundaries, and they're both very much minors without the full comprehension of the risks and challenges that might face them in these situations. Should their relationship break down, it could tear the family apart, and it could lead Nozomu to have to find another home and school etc.
While the author themself is very much a shipper on deck here (esp. re: their own NSFW doujins of the work), they're treating this budding romance with realism and maturity that we rarely get in the genre. Major respect!