Translation:My brother does the weather forecast.
It asks for "THE weather forecast" but there is not definite article before réamhaisnéis na haimsire. Is it always definite?
Réamhaisnéis na haimsire is a phrase with a definite genitive noun, which needs to omit all but the last article; despite this, everything in the phrase is definite. It literally translates as “the forecast of the weather”.
As an aside, the older spelling of réamhaisnéis was réamh-fhaisnéis ; I find that recognition of the components of a compound word helps me to remember how to spell it, and perhaps that recognition might help other people as well.