Wanda avenged her brother’s death when she destroyed Ultron’s main body, but Pietro wouldn’t be mentioned in the MCU again until WandaVision.
Sadly, Pietro perished when he ran in front of a hail of bullets to shield Hawkeye and a young boy from being struck.
However, once the twins learned that Ultron planned to wipe out all life on Earth, they joined Earth’s Mightiest Heroes in stopping him.
Along with Quicksilver’s speed obviously making him a powerful opponent since he could both run, think and act faster than normal people could perceive, his power also came in handy with evacuating Sokovian citizens during Ultron’s attack.