What is the best professional camera, for everything from weddings to sports, and for low lighting?

If you're talking pro cameras as in 'money is no object', probably the Nikon D2Xs. It has a practically bullet proof body with a 12.8 mega-pixel sensor that you can switch to 6.8 magapixels for 8 frames per second. Price aprox. $4500.
If you want the best in image quality and low light sensitivity, the Canon 1Ds Mark 2 is better. It has 16+ megapixels... but it can only do 4 frames per second. Price aprox. $8000. Canon sells the 'low-res' 1D Mark 2 N for professional sports photography.
With either brand, you'd want specific lenses for specific tasks. Top of the line zoom lenses and portrait lenses are all +$1000 each.

Not unless your relative is a pro-photographer. Otherwise forget about trying to save money on this and just pay a photographer. You could do this for a barbecue or family re-union, but not for a wedding. Weddings only happen once and there is only one shot at getting the pictures right. Pro wedding photographers must have the technical skills (which usuallt take years to learn), but they must also have a feel for people, and an understanding of how important their work is in peoples lives (frankly I think of it as a very scary profession since it's massive responsibility). If your willing to risk bad shots, then go ahead, otherwise get a pro and carefully study their portfolio before you choose them. You might also want to remember that if you buy a professional kit with all the right cameras, lenses, flash-units, it will probably cost you more than the photographer, and you'll leave your relative (the one called one to do the shooting) so bewildered by the eqiupment they'll become even more likely to do a poor job.

Here is a comparson of the top 5 10 meg DSLR's.

My choice is Pentax K10D (have been shooting with *ist D for over 4 yrs now, and upgrading shortly). But Nikons are good ones too (if you feel like spending more money)

If you've got the money, I agree with Ara57 and OMG. If you need to watch the budget a little bit, step down to the D200 without too much worry.

(It hurt to say "step down" to that camera, because it's a great camera.)

In general, you want one with excellent anti-shake function. I just studied the Canon Powershot S3. It has stablization mechanism that correct for 3 stops. That will allow good photos to be taken at low lighting condition.

It ain't just the camera, it's the lens, too.
I think the Nikon D2X has a pretty large buffer for taking quick consecutive shots for sports etc.
Canon seems to have the edge in low noise at high ISO.
Most "professional" cameras are built sturdier and have more weather sealing than the normal consumer level camera. Also, pro shutters are rated to last for more clicks.
For weddings, many pros use a fast zoom lens, rather heavy and pricey. Two camera bodies (at least) and ditto for flash units, brackets, diffusers etc. Sports photographers have long telephoto fast lenses, also heavy and pricey.
The real professional isn't the camera, it's the photographer!