What is the ruling on what I did, which was removing the niqab in front of a non-mahram man to avoid misunderstanding, knowing that it is obligatory, and am I considered sinful, and what should I do when I face this situation again with my father's relatives?

You must cover your face from non-mahram men, whether they are relatives or not, young or old, as long as you believe it is obligatory and Allah has opened your heart to it. It is not permissible to be lenient in this matter because of relatives' negative assumptions, for this is not an excuse to abandon an obligation. Strive to invite your family to follow the rulings of Sharia with gentleness, good manners, and kind treatment. Inform them of the words of scholars and some admonitions, while seeking help from Allah and relying upon Him.