So I just finished reading the one shot Onslaught Marvel Universe which can be seen as the climax in the whole Onslaught event:
However Onslaught was far from defeated, now in a purely psionic form, Onslaught had the power to spread all over the universe. Reed Richards determined that non-mutant vessels could absorb Onslaught's power leaving him open to be destroyed. Mr. Fantastic, Invisible Woman, Thing, Human Torch, Captain America, Iron Man, Giant Man, Wasp, Scarlet Witch, Black Panther, Namor, the Falcon, Hawkeye, Bruce Banner all willingly entered into Onslaught's energy form, and Dr. Doom was forced into it when he tried to capitalize from the power.
Shamelessly copied from the summary on marvel.wikia
But what I don't understand is why:
Reed Richards holds back Wolverine and Quicksilver because they are mutants but it is no problem that Scarlet Witch goes? She and Quickie are brother and sister (although this is later retconned I believe) and if one is a mutant, then the other one should be on too?