so that makes them a brother and sister through father only (a half sibling) which is permitted for bnei noach
First of all regardless of what was supposed to happen, Shimon and Dinah were full siblings. Secondly do you have a source that half siblings are permitted for Bnei Noach?
Anyways, only Yosef held that they were Bnei Noach, the rest of the brothers (at least the first ten, not sure about Binyamin) held that the Halachos of Bnei Yisroel applied to them.