How big chance it is to get a twin in sims 4?

I don't know...

Maybe this helps:

Influencing the Pregnancy
The pregnancy will typically produce a single baby. To increase the chances of twins or triplets, use the Fertile Reward. It's available from the Rewards Store for the price of 3,000 Satisfaction points. A patch made it so that we can influence the gender of babies:

• To have a baby boy: Eat carrots and listen to alternative music on the radio.
• To have a baby girl: Eat strawberries and listen to pop music on the radio.

Doing both of these things during your pregnancy will increase your odds of having a boy or girl.

We are unable to influence the rate of twins or triplets beyond the Fertile Reward.

Source: http://www.carls-sims-4-guide.com/parenting/babies.php