Countess Ostrida Regenwaldsdottir - Evidence for her being mother of Rognvald?

I don't know a source. Common to see her as wife of Brussi and mother of Rognvald. I don't think I've seen her as daughter of this couple before.

I think her patronym comes from Ragnvald the Old, Earl in Västergötland so not being suggested as a daughter of above. (Unless I'm misunderstanding you Justin)

=Common to see her as wife of Brussi and mother of Rognvald= Is it common to see Rognvald as a sibling of her other children too?

Dunno about that. Now you're getting into questions I don't know from memory. I'd actually have to poke around a bit ;)

To clarify:

I don't think I've ever seen parents for her. By "this couple" I meant the Geni parents. She could be daughter of any Ragnvald, even one who is unknown. I'm skeptical about this identification without a source.

I don't have any children for her except Rögnvaldr Brúsason, but that doesn't mean anything. I might not have been interested in any others.

Anything to do with Brúsi Sigurðarson is suspicious (to me) because of all the forgeries around him that tried to make him the eponymous ancestor of the Bruce family.

Okay, thats helpful. I'm going to leave Ostrida Ragnvaldsdotter as mother of Ragnvald II Brusasson, Earl of Orkney as well as the 3 other Brúsason siblings; and add a Curator note about it just being a commonly accepted relationship, but unverified.
I'll add a link to this Discussion - so if anybody in years to come wants to leap in with more info, they can.

I will start a new Discussion asking for corroboration of her parentage as Ragnvald the Old, Earl in Västergötland.