Both my younger siblings pronounce "thank" using ð, voicing the "th". I have never heard any other native speaker pronounce it this way. Both my parents, my older sibling, and I all pronounce "thank" using θ, not voicing the "th". I am really wondering where they got this pronunciation form from. From what I have found, the standard pronunciation of "thank" is always with θ.
Has anyone else ever heard "thank" pronounced using ð? Are there any English dialects in which this might be correct?