So here are my thoughts:
I immediately think about the interconversion of glutamine and glutamate. They convert in a loop cycle (1 converts into the other), so raising L-glutamine levels in the body will also have the potential to increase the pool of glutamate. Glutamate can be neuroexcitotoxic, if it cannot be converted into other things, mainly GABA (neurotransmitter). This requires GAD (glutamic acid decarboxylase).
You don’t need to have GAD antibodies to be simply not great at the conversion. There is variability there, one person to another, based on genetics.
The negative effects of glutamate stimulation can be reduced with magnesium and glycine, so giving magnesium glycinate daily with the glutamine may help your sister.
This is not a glutamine allergy (IgE response) or sensitivity (IgG response). It’s a biochemical process in the body that isn’t antibody based.