What are the chances of my baby brother dying if he caught the real influenza?

Not much, but maybe. How old is baby brother? The odds are that he will recover uneventfully but may be quite uncomfortable for a week or so. Your mom will watch closely and get good medical care if he gets badly ill, so help her out and be supportive. Hope brother is better soon.
It happens. I almost lost an otherwise healthy 3 yo 2 yrs ago in spite of antivirals started the first day of symptoms.She spent a month in the ICU and got as close as any patient I ever dealt with that finally survived.That was the only such flu case like it I had in 3+ decades of practice. Flu is a disease I respect and worry about, but death is rare among the healthy population.