Is the word quem the same in Brazilian and European Portuguese?

No.

While the spelling and meaning of "quem" are identical in both Brazilian and Continental Portuguese, there are differences in pronunciation and typical usage.

In terms of pronunciation, the nasal diphthong in "quem" is generally more open and clearly articulated in most Brazilian dialects, whereas in Continental Portuguese, the vowel sound is often more closed or subject to the heavy vowel reduction characteristic of the dialect.

In terms of usage, while "quem" is used in both dialects to refer to people, the way it functions within a sentence varies. In Continental Portuguese, speakers strictly adhere to using "quem" (or "que") with a preposition whenever the verb requires one (e.g., "de quem"). In Brazilian Portuguese, especially in informal speech, there is a strong tendency to simplify these structures, often replacing "quem" with "que" and omitting the necessary preposition (e.g., "a pessoa que eu falei" instead of "a pessoa de quem eu falei").

Brazilian Portuguese

1. Quem é você? (Who are you?)
2. Eu não sei quem ligou. (I don't know who called.)
3. Quem quer pizza? (Who wants pizza?)
4. Eu não sei de quem você está falando. (I don't know who you are talking about.)
5. Quem vai na festa? (Who is going to the party?)

Continental Portuguese

1. Quem és tu? (Who are you?)
2. Não sei quem terá ligado. (I don't know who might have called.)
3. Quem deseja pizza? (Who wants pizza?)
4. Não sei de quem tu falas. (I don't know who you are talking about.)
5. Quem irá à festa? (Who will go to the party?)