Now, I'm completely new to puzzling.SE, so maybe I'm missing something, but the answer here seems remarkably clear to me:
No one killed Mike.
Mike's unnamed older brother says that three people present are lying. So the trick is to find the three people whose statements, when treated as false, result in there being no contradictions among all the statements we've received. So here we go!
Liar number one:
Inspector says that Mike was killed. But in truth, Mike is completely fine. Look; Mike just spilled a little wine on himself and went upstairs to change his shirt. He'll be right back down. You really need to calm down, dude; I think your job is starting to get to you.
Look, you made Cindy cry.
Inspector is the first liar.
Liar number two:
Okay, I know what you're thinking.
You're thinking this answer is already completely invalid, because @athin told us in a clarification that Inspector isn't one of the liars. And you would be correct, except that @athin is also a liar.
As proof, I need only point out that @athin says that I am an investigator.
But in truth I am not an investigator; I am a chartered accountant. Therefore, @athin is liar number two, and the clarification claiming that Inspector isn't a liar proof-positive confirms that Inspector is, in fact, one of the other two liars.
That brings us to liar number three:
Okay, I know I said that I'm a chartered accountant, but.. well.. you caught me, that was a little bit of a fib. I am actually a professional orange-taster. I'm very sensitive about it and don't like to talk about it, but you and your accusatory stabby eyes pried it out of me.
I hope you're happy. Brute.
Anyhow, I guess if you're going to be pedantic about it, I'm not actually new to puzzling.SE; I've been here for a couple years, mostly lurking over in the lateral-thinking puzzles. I only ventured out because of the open bar at the wedding reception.
Oh, and while we're being completely honest with each other, I guess I should confess that Mike doesn't actually have an older brother; I just made him up and claimed he was my friend to get into the reception and the open bar. I'm kind of shocked that fooled any of you; I mean, I didn't even give him a name! I just kept referring to him as "Mike's older brother" and "my friend", and nobody asked any questions. I mean, what sort of rubes are you people?
So.. okay, I guess I'll cop to being liar number three.
The remainder of the comments:
Austin, Daniel, Cindy, and Bella all claimed not to have killed Mike. And as we've established that the Inspector was lying about Mike being dead, there is no contradiction here.
Additionally, Austin tells us that Daniel was jealous, and Daniel agrees that he was jealous, so there's no contradiction there, either.
Finally, Mike's older brother is also not one of the liars. That's in part because he doesn't exist (because I made him up), but also because his only statement was that three people were lying, which turned out to be entirely true.
Hope that clears things up!