We didn't have twins but we had 2 cats when our baby was born 11 weeks ago. I was very nervous how they would react- possibly jealous or too curious? But the girl cat just got bod on with me because I didn't pay her much attention anymore and our tom completely ignores her completely. Didn't make any difference at all to him! Now neither of them go near her really.As yinyang says, she's just a noisy nuisance.
We are always VERY careful not to let the cats go into a room where she is sleeping. I really don't think they would get in the crib but you can't be sure really. They will always try to get into the cosiest, warmest places. (Basket of clean white ironing!??)