Question: why are quicksilver and scarlet witch in the avengers?
Answer: Wanda and Pietro were altered in the MCU by Baron Strucker who gave them their powers so that they could get revenge on Stark. The "Mutant" trademark was sold to 20th Century Fox, who make the X-Men movies. The MCU got around this as they still own The Avengers franchise, of which both of the Maximoffs were members.