It would be great if everyone would just stop when someone says no, but it's unfortunately not that simple. Plus, if incest was legal and the girl is quite young she might be pressured into thinking she should do it. At least if it's seen as taboo or is illegal then she might be more inclined to say no, or tell someone about it.
wat difference is there between coerced sex between siblings, and coerced sex between non-related ppl?
there isnt
its just that one is "even more illegal" bcuz its incest. but again, ur misappropriating the law for that purpose. the incest law isnt designed to stop one sibling from coercing the other into sex.