Baby with viral infection and sibling with chicken pox

[iwasagirlinavillage]

DD2 is 7 months old (5 months corrected). She is small and has had what the GP has said has been a viral infection for almost 4 weeks now (cold, cough, conjunctivitis, swollen glands, red throat). She also has oral thrush which she's had for about 3 weeks. Neither seem to be clearing so I was getting a bit concerned anyway that her immune system seems to be a bit weak and now her older sister has come out with chicken pox. Obviously it's highly likely DD2 will get it as well, but should I be concerned with her already having a viral infection and her apparent inability to shift it?

Thanks