I do not see any problem here sex with half sister is nothing special you can even officially marry her
If you are both of the similar age and she is cooperative you can have some fun with her.
If you are both underage then great you have no responsibility for what you do, enjoy your freedom. you can have sex with anyone you want nobody can punish you for that.
If not well then there is no difference from any other girl next door. All you will get to do is to fap on her picture.
Just join army of the unfortunate guys who get into various celebrities or other girls whom they will never have