The Japanese are an emotionally stunted group of people due to the workaholic structure of their society. Incest is popular because those emotional needs are fulfilled in an unconditional way due to them being family. There might always be strings attached in a non-incest relationship, but an incest relationship doesn't, at least in fantasy. Each member of the family fills a particular emotional need, which then gets mixed in with sexual desire, resulting in an incestual fantasy. Imoutos are immensely popular because they make the person feel looked up to and loved unconditionally, no matter how flawed they are. In their fantasy, an imouto will always love their onii-chan, and the onii-chans in turn have someone to cherish. This is why sister incest is very popular with guys who don't have sisters.
Mothers fall into a different category. The desire for a mother can either be through a lack of maternal affection or from too much affection/doting. For those who received no affection or was possibly abused, the appeal of a mother comes in the form of wanting to be doted up and cared for unconditionally. Instead of having someone that looks up to them, mothers instead give them the nurture and attention that they never had. On the flip side, those that received too much affection suffer from an inability to let go. They become so dependent on their mother's affection that the prospect of letting go terrifies them, since there might no be someone who loves and cares for them so unconditionally like their mother.
Onee-sans fall into a similar category as mothers, with the guy wanting their older sister to care and nurture them in a similar way. And daughters fall into a similar category as imoutos, with the guy wanting someone looking up to them and attached to them.
All of this gets mixed in with sexual frustration or just excess sexual desire and boom, incest.