Boy-Girl Twins! (No rules to follow either)

[Cassie]

LOL I quite love Alien families in TS3. XD
They're so cute, the kids. :]

The way I get boy-girl twins is;
Have a Sim who's past level 6 [or is it 8?] in the Medical Career, for example Jamie Jolina in Sunset Valley would work, and move her in with your pregnant Sim. Get the doctor to choose the social 'Reveal Gender of Baby' and if it says you're having a boy, go to the grocery store and have your Sim purchase and eat 15-20 watermelons. If it says you're having a girl, do the same, except swap watermelons for apples.
Oh and you can now move the Doctor out, you're done with him or her.

And obviously make sure you're having twins too, by either having the Fertility Treatment Reward before your Sim gets pregnant, or reading the pregnancy books, listening to the Kids Radio Station and watching the Kids Zone channel.

Reads complicated, it's easy really, . :]