I feel so angry and helpless about people using fanart with no respect for the cultural context, especially since it's so often used to promote different agendas.

A few people already wrote to tell them.

This is kinda offensive several times over - stealing art and fan culture, changing the agenda, and it being so obvious they wouldn't promote a consensual incestuous relationship - it's like if a wedding site accidentally used a picture of a butch/femme lesbian wedding - how offensive would the horror at that be.