Why do you assume Tobirama has to be Hashirama's level just cause they are brothers? Madara and Izuna are tiers apart from each other.
-Hashirama spiked up his chakra and almost destroyed Naka Shrine, all to stop Tobirama from attacking Taka.
-Hashirama gave Tobirama "the look" when he was about to kill Madara, guess what happened. Tobirama stopped.
-Hashirama broke his restraints implemented by Orochimaru. Tobirama couldn't.
Hashirama was seen as a fairy tale during his era, his might was so vast that people didn't even believe he existed. Tobirama wasn't.
There is a gap between them and it is very large. Tobirama is going to end up being around Minato's level, either a little weaker or a little stronger, from the looks of things Tobirama is probably going to end up being a little weaker than Base Minato and much weaker than KCM Minato.
Hashirama
Minato
Hiruzen (Prime)
Tobirama
Tsunade