Is refraining from advising the relatives of paternal uncles who commit mistakes, for fear of their ridicule, considered a sin, knowing that I advise my older siblings who do not respond to advice?

If you are committed to obeying Allah and inviting people to Him, then be keen on the virtue of patience regarding what befalls you on this path, as Luqman enjoined his son: "And be patient over what befalls you. Indeed, that is of the matters [requiring] determination." By doing so, you will be saved from loss, as stated in Surat Al-Asr. You should also seek beneficial knowledge, adorn yourself with good character, and respond to evil with goodness; for that transforms animosity into affection, "But none is granted it except those who are patient, and none is granted it except one having a great portion [of good]."