Assalamu Alaykom,
May Allah reward you & us jannah al ferdaws.
I know that jealousy between the spouses is something normal but for the benefits of others pls explain to me if it is SHIRK because a husband forbade his other wife for getting jealous because he said it is shirk. is it so?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.

What that person said that jealousy is Shirk is not correct; for more benefit on the definition of Shirk, please refer to Fataawa 82456 and 85291.

A wife being jealous of the other wives of her husband is something natural as the wives of the Prophet used to be jealous about him.

However, if jealousy exceeds its limits of moderation, then it is dispraised especially if it leads to something forbidden like injustice and transgression; for more benefit, please refer to Fatwa 90757.

Allaah Knows best.