I found out after they were born, I made 3 different decorations ( twins girls, twins boys, mixed twins ) and took the pics.
It’s really hard to have a gender reveal party in the sims 2, especially with twins ( with a single baby it’s possible to save just before the birth, and quiting without saving if it’s not t(he wanted gender ).
It’s easier in the sims 3 and very easy in the sims 4 with the mc commander.
I was sure Loulou was having two girls or a boy and a girl, I never had twins boys in the sims 2.