It’s either tears of relief because she now knows she’s not alone or tears of sadness because she got step sibling-zoned. I’d say it’s a bitter one either way.
I think this just cemented the feelings. But good on our boy for actually being genuine about considering them as siblings and despite knowing they aren't they will still be together
4D chess plot twist: they are in fact all related but just not sibling related and this was all just an experiment to see how they would react with the news.