Does seeing dead relatives in a dream, speaking and asking for charity on their behalf, mean that these are truly their souls, and must their requests be fulfilled?

It cannot be asserted with certainty that the soul a person saw in a dream is the soul of a specific individual, even if that is generally possible. As for fulfilling what was requested, if it is something permissible, like giving charity, then it should be fulfilled as a recommendation, not as an obligation.