Hi Daan. Has your niece been diagnosed with CLS and is she receiving treatment with IVIG. If neither of these had happened as yet it needs to happen as soon as possible. You will see from the resource material produced by Arturo that Dr Druery at NIH in Washington DC is a critical resource to confirm the diagnosis.
She also needs to be treated with IVIG.
When she is having an attack it is important that fluids are introduced in a measured way and that a fluid is used with a large molecule.
All of this information with more detail is contained in the resource section which you should point your Drs towards. Warren