@Omnya
Asalam Aleikum
Salaanta ka dib
It's a sticky situation tbh but the real reason why the parents are ok with their son marrying out is due to lineage, his children will carry his lineage and will be always be considered somali but with their daughter the children will not, as lineage is passed down through the father in islam. Lineage is one of the qualities that is sought after in marriage and in islam this is allowed.
Correct me if i'm wrong but it seems like the sister has been in touch with the revert brother for quite some time and is only deciding to inform her wali now after she vetted him herself,? enough time has gone by for her to develop feelings for this brother which further complicates this issue all together. Even if the sister had acted according to islam and informed her wali about the revert so as to let the wali do his job chances are he still would never accept him solely on his background never mind the issue with having non-muslim in laws etc
Only solution now is to go ahead and inform her wali about the brother and should they decide to reject him then best thing to do is move forward and forget this brother as it's only going to create more drama down the line tbh. I know it'll be hard but it's better than the alternatives out there
I just hope that she doesn't focus on her parent's rejection due to his lineage and say it's "haram/un-islamic" when she has interactions with a non mahram without the consent of her wali to begin with. Had she cared about what islam says then she wouldn't be talking to the revert brother in the first place, so how come rules didn't matter then but do now ?
We're all walking contradictions, best course of action is to focus on our's before we turn to others.