You can be an owner and/or investor of more than one business. However, you want to be sure that your obligations to one business (e.g., use of machinery, understanding of confidential information) don't conflict with another business.

If you are not using any of the machinery, information, or resources of your cousin's company, and if your new ventures will not compete with his, you have no obligation to give him an interest in your other activities. If he is putting in no time, effort, capital or resources in connection with your other ventures, there’s really no need to give him more than a minimal percentage, if any.

Best to speak to an attorney and an accountant about how to structure and value the interest you feel is fair for him to have.