i am trying to figure out how my Dads Dna is saying someone is his brother?

Geni pulls this data directly from FamilyTree. You'll want to inquire them to validate no mistake was made.

Private User -- one does not upload anything from FTDNA to Geni. One links the accounts.
Geni goes thru looking at the profiles that have linked FTDNA and Geni looks for those that have done that and match at FTDNA. Geni takes the cM shared and longest block from FTDNA, But Geni uses its own algorithm for the relationship range.

Look on FTDNA. What does FTDNA say for the suggested relationship?
Overlap: 1823.5 cM is a lot. Longest segment: 128.1 cM is a significant amount.

This may be helpful. https://blog.kittycooper.com/2016/08/how-to-tell-the-relationship-f...

I dont see Mr. Stefan Simander listed, He is only Listed on Geni. How is that?

He may not be using the same name. Which folks, if any, do you see listed on FTDNA who share over 1800 cM with your Dad?

Also, looking at your first post again -- where is that from??
Is it from your DNA matches Tab, or from comparing your father and a DNA match?

The more I think about it, the more I notice that it does not read like it is from the DNA Tab,
And wonder if you are misinterpreting a page starting "Autosomal DNA match between you and ..."
With "you" being your father and it has a table, with shared matches to the two down the center
To the left, how that shared match matches "you" --- to the right, how that shared match matches the other person -
Totally possible that Stefan might be 3rd to 2nd Cousin Twice Removed Overlap: 50.1 cM
Longest segment: 7.7 cM with your father but be
Brother - Overlap: 1823.5 cM Longest segment: 128.1 cM to the other person, who is a match of your father