While there are similarities between Brock & Hank, Hank was (first) born before Brock became Rusty's bodyguard and we're led to assume they never met between then and college. I think it's more likely Myra is definitely the Venture Brother's mother and Hank simply takes from his father's lack of coordination and his mothers hair, aggressiveness, and crazy.
While I'm at it, I don't think Dermott is Brock's kid either, he looks a lot more like Sgt. Hatred and he's somehow less coordinated than Hank. Further, he's a habitual liar (or at least boasts about shit he never actual did and then tries to make excuses for it), so when he said to 24 "Would you believe I'm Brock Samson's son?" I'd have to agree with 24, since he really shouldn't have taken a mini-Brock down so easily and we know he makes shit up all the time. Also, the day he said "Are you sure he's my real father?" Sgt. Hatred was there.