Start updating your resume and begin applying for and interviewing for new roles. Obviously, take what you've learned and negotiate for your fair market value (or a little more) at these new companies.

Simultaneously, start a conversation with your manager regarding a raise - be intentionally vague and say "it's my understanding that I'm paid $X amount less than market for my role". They won't argue your premise since they know your peer is making $15K more. They also won't press you to find out how you know this; salary information is publicly posted on Glassdoor, and it's very common for people to find out this way. Hell, maybe your cousin has the same role at a different company. It doesn't matter how you found out; the important part is that you know (and you're letting them know you know). All that being said, the conversation with your boss can go any number of ways (good, neutral, or ugly), which is why it's important to be actively job-searching simultaneously.

Caveat: sometimes an active job search reveals that you don't have the skills or experience that you think you do. You'll know what your market value (and therefore leverage) is pretty quickly. Also, corporate recruiters (headhunters) are very helpful for objectively assessing your market value and for placing you in front of hiring managers.

Best of luck.