I would argue that, like child marriage, instances where sibling marriage does happen can be tied back to abuse. Inbreeding aside, sibling marriage can come in several forms. If it is kept illegal, it gives the state the excuse to investigate and break manipulated young girls from oppressive family structures.

Take FLDS, where they forcibly married girls underage to their cousins. Or they removed girls altogether from their homes with the consent of their parents and trapped them in a giant Texas compound. The girls knew nothing else, and were sexually abused by much older men.

Edit: Last thought: most sexual assaults in childhood are by family or friends close to the family.

So, banning sibling marriage may be a qualifier to stop low quality of life for the women that end up in the marriage, very similar to how loitering with intent to do prostitution allows police to question and find trafficing victims.