From what I understand, if this "Predator Spirit" in half-Reds were the same as being a full Red, then Susan or Martian could have been sacrificed anytime.
The Sacrifice only worked because Susan fed her RC Vampire and let it take over. 'Susan' died as soon as she fed, and a brand new RC Vampire was born. At that moment of birth, she became the youngest Rampire and could affect all others.
There is no evidence that the RC's Predator Spirit infection that turns a human into a Rampire can be passed down any other way except from a RC Noble (the process of doing so isn't entirely clear).
It is unlikely that Maggie would have inherited any of that RC Infection, if she had her first foster family would have all been dead soon after they took her in. A child would not have been able to control that. Also, when the Rampires were all killed off, it also wiped out the 'Infection' that made Half-Reds. Even if Maggie had any of that 'Infection', it would have been wiped away once Susan died.