Longer answer: In modern terms incest is almost always by definition illegal relations with a family member (your fifth cousin is technically your family but far enough removed from you that there are no laws against fifth cousins marrying, so it's not classified as incest) so unless it's illegal to masturbate where you live, then it probably wouldn't be considered a form of incest. Psychologically, incest is usually recognized as happening because of one family member grooming or otherwise abusing the other into the relationship, and you literally can't groom yourself... I know you can self-harm, but erm, I'm pretty sure you can't technically abuse yourself into having sexual relations with... yourself. So probably still no.
no subject
Longer answer: In modern terms incest is almost always by definition illegal relations with a family member (your fifth cousin is technically your family but far enough removed from you that there are no laws against fifth cousins marrying, so it's not classified as incest) so unless it's illegal to masturbate where you live, then it probably wouldn't be considered a form of incest. Psychologically, incest is usually recognized as happening because of one family member grooming or otherwise abusing the other into the relationship, and you literally can't groom yourself... I know you can self-harm, but erm, I'm pretty sure you can't technically abuse yourself into having sexual relations with... yourself. So probably still no.