The decision to show your hair to anyone, including a double cousin (also known as a second cousin), is a personal choice and depends on your beliefs, values, and cultural practices as a hijabi. In Islam, the concept of hijab varies among different cultures and interpretations, and what is considered appropriate can differ from one person to another.
If you have a close relationship with your double cousin and you both feel comfortable and respectful of each other's boundaries, you may choose to share your decision with them and ask for their understanding and support. Open communication is essential in any relationship, and discussing your feelings and beliefs can lead to a better understanding between you both.
Remember that your choice should align with your personal beliefs and comfort level. Your decision to show your hair or not should not solely be based on the degree of blood relationship, but rather on your values and the understanding you have about hijab in your religious and cultural context.
If you have any doubts or concerns, consider seeking advice from a trusted religious scholar or counselor who can provide guidance based on Islamic teachings and your specific circumstances.