....”As a Catholic, I consider protestant Christians to be my brothers and sisters in Christ. But the anti-Catholic nastiness on this thread does not come across as very Christian.”.....
Perhaps you need to refresh yourself on what the Lord requires of us in opposing false doctrines and traditions taught by men.
As A Follower of Jesus Christ I do not assume any are fellow Christians as the Holy Spirit is faithful to bear witness of who they are....some mask as so but are not. Others know scripture but use it to justify their position rather than what it clearly states. We should always be under the influence of the Holy Spirit in discerning what is true and what is false....not what our church necessarily says.
As for nastiness...throughout History believers have engaged in debate which can and does get heated...that’s natural and it will always be so.
However...there are other threads you might be more comfortable in that do not get so heated. I find these always teach and learn and strengthen my faith in what is truth and falsehood.