You shouldn't have built the foundation of your career on deceit. You'll likely be given the employment without experience if you're good enough. You can still make your way right and have them guide you. Everybody starts somewhere.

Enrolling in the college of immorality?

But whoso committeth adultery with a woman lacketh understanding: he that doeth it destroyeth his own soul. A wound and dishonour shall he get; and his reproach shall not be wiped away.
-Proverbs 6:32-33

Please do not look away! This is your business. What's going on is very abominable if you're right at all, and if nothing is done it will come back to haunt your family.

If you have an elderly brother, make your suspicion known to him. If not, your mother needs to know. But keep it away from your cousin's mother and from the extended family.

Maybe nothing has happened between them, but you have enough reasons already to be inconvenient.

Please approach this matter wisely and prayerfully. Help your brother before he ruins himself.