I think he's probably properly rated, but given his brother's rating, I think you could argue he's a little underrated.
Brady is an absolute unicorn when you look at power forwards who play physical, get pucks to net, and produce offense. I've said for years that, in a vacuum, he's a better all around player than his brother, though it's really close, and could go either way depending on team needs.
Matthew has shown to be the more skilled playmaker and more opportunistic goal scorer - give Matthew an elite puck transporter on his line (Gaudreau, Bennett) and it's obvious seen how effective he can be.
Since 2020-21:
Brady
5v5 rates
0.96 g/60
2.10 p/60
1.79 p1/60
Misc rates
11.71 hits/60
2.11 GvA/60
1.61 TkA/60
Matthew
5v5 rates
0.99 g/60
2.58 p/60
1.98 p1/60
Misc rates
4.93 hits/60
3.16 GvA/60
1.55 TkA/60
I think if you switched their situations, their production rates probably converge to nearly equivalent. Matthew is the better pure offensive talent, but the disparity in their team situations has made the gap look larger than it really is, while Brady is the better goal scorer and more impactful all around force.