hello and welcome to HCM,
In all probability your sister seems to be suffering from an infection. But in the background of the lump in her neck and the persistent stomach pain you need to definitely seen an ENT who can give you a proper diagnosis and treatment.
meanwhile avoid spicy and cold
Do not try to palpate or meddle with the lump
Do not try to self medicate
when in birth has not much relation to this problem except probably in her medication
take care and get well soon
Dr Sriram Nathan