They arent related by blood.
They were both from the same group home, by Bob Shamrock.
Bob legally adopted Ken and Frank, and they both adopted his last name: Shamrock.
Then, Ken started doing MMA, and Bob got Frank to train with him.
Frank learned from Ken, who was good at MMA And then after learning from Ken, and getting a good name, he abandoned the Lions Den and went on his merry way.