It seems like you are dealing with some complex emotions and social dynamics. It's important to approach the situation with honesty, sensitivity, and open communication. Here are some steps to consider:
Self-reflection: Take some time to understand your feelings and why they may have changed. Reflect on why you initially said no and why you now feel differently.
Evaluate your intentions: Ensure that your feelings are genuine and not influenced by external factors, such as peer pressure or social expectations.
Communicate with your sister: Talk to your sister openly and honestly about how your feelings have evolved. Be respectful of her feelings and listen to her perspective as well.
Talk to the person involved: If you feel comfortable doing so, have a conversation with your sister's boyfriend's cousin about your feelings. Be genuine and sincere in expressing yourself.
Be prepared for the outcome: Understand that the other person may have moved on or may not feel the same way. Be respectful of their feelings, just as you would expect them to be respectful of yours.
Respect boundaries: Regardless of the outcome, respect everyone's boundaries and personal choices.
Learn from the experience: Use this situation as an opportunity for personal growth and understanding of your emotions.
Remember that relationships can be complicated, and it's essential to handle them with care and sensitivity. Always be true to yourself and communicate openly and honestly with others involved.