Ok Wanda, Pietro and Magneto's backstories have been rewritten so many times I have a hard time following it.
What surname did Wanda and Pietro have when they joined the Brotherhood of Evil Mutants?
As I understand it the family history Magneto (Max Eisenhardt/Erik Erik Lehnsherr) married Magda and after Auschwitz they settled down married and had a daughter named Anya who was was killed in a fire. Magneto was prevented from saving Anya because the towns people found out he was a mutant...when Anya died from the fire Magneto flash fried the flash mob with his powers leaving a pregnant Magda horrified. Late in her pregnancy Magda made her way to Wundagore Mountain. Robert Frank (Whizzer) and a heavily pregnant Madeline Joyce Frank (Miss America) had also recently came to the mountain. Magda gave birth to Pietro and Wanda and died. Madeline gave birth to a stillborn baby and died. Bova tried passing the twins off as Madeline's childen to make Robert feel better but he bailed and took off running. So later the High Evolutionary gives the twins to Djano and Marya Maximoff who are Romany gypsies. The couple had recently lost their own children Ana and Mateo...and this is sort of where my memory starts to fail.
As I understand it Wanda and Pietro started using the Frank surname after they came to the conclusion that Whizzer was their father. After becoming reacquainted with their foster father Django they started using the Maximoff name (again?).
So to me there appears to be a gap from the time they were initially raised by Django and Marya and when they were recruited by Magneto into the Brotherhood. They didn't remember Django when they saw him again but as I understand it Wanda and Pietro were either teenagers or young adults when Magneto recruited them.
So how did they get separated from Django? Why didn't they remember Django? What surname were they using after they were separated from Django but before meeting Magneto? Were they just two amnesiacs who for years just couldn't remember their last names?
The answer you want is basically that they didn't have a last name. Wanda and Pietro were never given a last name by Stan Lee, and they existed in the X-men stories as part of Brotherhood and in the Avengers for a long time before a writer came up with an origin, much less a last name. The idea of being the children of the Whizzer came along (an avengers annual storyline, after which the last name of Frank started being used) before Django (by name) was introduced in the Avengers (basically the yesterday quest storyline). And then the Magneto factor was added later.