Here me out. This is an alternate route 

First of all. Inbreeding or incest is considered taboo in many cultures (i mean all of them, if u excuse the *royal* bloodlines of history)

If Megumi developed this feelings. He would know it is forbidden ,but due to daddy neglect ,he will retain this feelings throughout the year

Slowly and slowly, he develops his cursed technique while also slowly brewing this feeling of love he has for his sister.

He shuts himself off from her sis about his actual feelings.and the only way he shows his affection is through wearing her printed t shirt everywhere.

He takes a binding vow for his siscon issue. More he thinks it's disgusting and morally wrong

Also judged negatively by adults and others for his obsession with her

Those negative emotions turn into 
#cursed energy

Since this is outside of his feelings too. It might be possible he would even surpass rikas in cursed energy threshold 

With boost in shikigami's too.


If Sukuna tries to take over Megumi's body, he will realise how much Megumi is obsessed about Tsumiki 


He will beg Yuji to take him back.

The amount of CURSED ENERGY HE CAn tap into is Immeasurable.

Even sukuna or gojo wouldn't pull off this kind of technique

Only Wegumi Himshiguro can pull this off


If Wegumi was a siscon, he would be the strongest sorcerer, surpassing gojo and SUKUNA