Learn French: Leurs prévisions sont rarement fiables. - Their forecasts are rarely reliable.

In French, possessive adjectives agree in number with the thing being owned, not the owners themselves. Since the noun prévisions is plural, you must use the plural form Leurs. If they only had one forecast, it would be Leur prévision.

Is prévisions used for weather forecasts or business forecasts?

It can be both! Prévisions broadly means 'forecasts' or 'predictions'. It is heavily used in business and economics, but it is also the standard shorthand for les prévisions météorologiques (the weather forecast).

Why does the adverb rarement come after the verb sont?

In English, adverbs of frequency often move around (like 'rarely reliable'), but French sentence structure is stricter. An adverb like rarement usually goes immediately after the conjugated verb it modifies. That's why it is sont rarement.

How does the adjective fiable change depending on gender?

It doesn't change! Because the singular form fiable (reliable) already ends in an -e, it is the exact same for both masculine and feminine nouns. Here, it just takes an -s to agree with the plural noun prévisions.