Of course they can, but they have to have a VERY high relationship status. You can also use the spell but it doesn't work well on other sims who use magic.
they can get married they have to be 4 or more happiness to propose and you have to have a lot of liking in the other sim.
There is a spell, third page in spell book 3rd one down (i think), its called the big question work everytime unless cast on another magic sim. Or you can do it the long way , be in love, a good mood and propose!