No, they won't need to be there. You see, sims get memories of their relatives getting engaged and married, even if they don't attend their weddings. To fulfill the lifetime want, that sim don't have to be in the same household. If you do that and return to that sim's family, you'll see that the sim has the memory of fulfilling the lifetime want and have the aspiration bar be on the platimum and stay that way (Fulfilling a fear will still decrease it, but I'll return to the platimum after some time). What will you also notice is that sim gets a new lifetime want.
If you get a career one and the sim is an elder, you can't fulfill it as elders can only join the part-time careers and only have full-time careers if they had one in their adulthood (so they can keep going to work or retire).
Oh, and your sim will also get a lot of aspiration points. Check the Aspiration Reward store (chest icon) to see what you can get, but do remember that these rewards may backfire when your sim uses it with low aspiration bar.