Hey Mark,
at first, berliner is capitalised small, as well as adjactives refering to countries (deutsch, englisch,...)
Then; wem, or as you said, -em is not genitive but dative.
And the easiest way to keep this in mind or remeber what form you need is to ask. In German:
WER bekommt eine DVD über die berliner Clubszene? - Mein kleiner Bruder.
WEM gebe ich einen berliner Aschenbecher? - MeinEM großen Bruder.
This is quite hard to learn for native English speakers, but keep it up. There will come a moment when it makes click and you got it with asking for the case.
Sincerely, Timon. :-)