As @EugeneT says, demand is a very strong word; generally it would only be used in a negotiation between neutral-to-hostile parties. (When two warring countries sit down to negotiate a cease-fire, for example, they might each bring a list of things they will demand as part of the settlement.)
If you are making a demand, generally there is an implied (or explicit) ultimatum behind it: "You give me what I demand, or else I will cause some unpleasant consequence!"
So you probably want to use a friendlier phrasing most of the time, and especially in this situation. Some suggestions:
I need to get a copy of the birthday video from you.
Can you send me a copy of the birthday video?
Please send me a copy of the birthday video.
2This question appears to be off-topic because OP has implicitly answered his own question in the first sentence (by saying that what he wants is *a video*, rather than *the video* or just plain *video*). The jocular use of *demand* is irrelevant. – FumbleFingers Reinstate Monica – 2013-12-26T20:12:26.697
If you want to remain friends, you'd probably rather use more polite words than demand when you request the video from her. – The Photon – 2013-12-27T02:33:56.513