I found a discussion thread about this in the Sims 3 Forums, but the original article was from SimTimes. Here is the article exert (loosely translated from German):
Did you create your Sims, you can in the relationship now, the options “Select Fixed friends Festivals friend” and “fiance / fiancee.
The just of the article is that in CAS you can make your sims go steady, be best friends or fiances. This should be a great feature to use.
Finally! They should have implemented this on The Sims 2.
=)
It’s an improvement, but I’d like to see Step-Parent and Step-Child added, as well as Half-Sibling.
It’s irritating to have to create two families, then break them up and get an adult from each family to marry each other just to create a family with step/half relatives.
I agree with Ashleigh825… we need more options to avoid the hassle of certain planned relationships. But this is definitely a start.