Rather than just a small amount of money given to various relatives I wish you could choose where YOUR money goes when you die and who to, so basically have a will.
Are there any mods related to this kind of thing? Inheritance and whatnot?
I know you can get rid of it all together but that's not what I want.
Especially if I have a sim with like 100k and all of that will just disappear if there is no one living at his house when he dies.

You can use the "Give gift" interaction to pass items with the correct value along before you die. I'm not aware of any mod that has a will functionality, though.

You can also use no inheritance so there is no automatic money handed around. http://modthesims.info/d/288785 I don't know if it still works.

I have that no inheritance mod and it works. Much better - no random money coming out of the sky with no knowledge of where it went to.

It tends to be elders who die totally alone, and you can tell when they are close to dying of old age, so I've developed something of a tradition - a day or so before death, the elder in question invites all of their closest friends around for a bequeathing. I stuff any sentimental or valuable belongings that I don't want left in the house into inventory, and they give them as gifts to other sims. Pets get transferred the same way. I make a note of their household worth (in build/buy mode, click on the i in the top left) and household cash and write it down on a little notepad. They die alone, perhaps with a relative or friend visiting. I don't think it's a good idea to have a party happening on the lot when the last sim dies because I think the party controller could get stuck there and cause problems. Then once they are dead and I have tidied up the lot for the next resident, I use the familyfunds cheat to "pass on" the inheritance to the family members and friends I choose.

You could also have them sell their house and move in with a friend/relative just before they die (or have the relatives move in with them ie "inherit the house") and then use the Money Order or another money-moving method for the friend/relative to spread the inheritance around anybody else who gets it.