Learn Portuguese (Portugal): A solidariedade na instituição é maior.

While caridade refers to the general concept or act of charity, an actual charitable organization or non-profit is usually called an instituição in Portugal (often short for instituição de solidariedade social).

Does maior change its ending because solidariedade is feminine?

No, maior (meaning "greater" or "bigger") is the irregular comparative form of grande. Just like grande, it only changes for plurals (becoming maiores), but stays exactly the same for both masculine and feminine nouns.

Why do we use é (from the verb ser) instead of está (from estar) here?

We use ser because the high level of solidarity is being described as a permanent characteristic, trait, or established fact about this specific charity, rather than just a temporary passing mood.

Why does na translate to "at the" here instead of "in the"?

The Portuguese preposition em (which combines with the feminine article a to make na) can mean "in", "on", or "at" depending on the context. While English usually prefers "at the charity", Portuguese simply uses its versatile preposition em.