Is it permissible to take money from a shop's income without the owners' knowledge to meet a need for food, and is it obligatory to return this money or inform the owners?

Your work at the company is a private lease [contract of hire], and you are obligated to be trustworthy in your work. Therefore, it is not permissible for you to take anything except with permission, for Allah Almighty says: "O you who have believed, do not betray Allah and the Messenger or betray your trusts while you know [the consequence]."

It is not permissible to purchase meals from the company's funds unless there is explicit permission or a known custom; otherwise, it would be consuming wealth unlawfully, for Allah Almighty says: "Do not consume your property among yourselves by unlawful means."

If you have taken from the company's funds what you are not entitled to, you must repent and return the money to it, even if indirectly, or seek their forgiveness.

As for the promise your brother-in-law made to you, if it was an agreement on a specific salary that he did not give you, then you have a right to it. If it was a promise to increase your salary that he did not fulfill, you may demand that he fulfill it. However, it is not permissible for you to take anything from his money without his knowledge based on this promise.