You jerked off and felt guilty, means you know that is wrong, right?
What happens accidentally, lets keep that way. Dont think of anything, because she was also nude in your mother's womb where you were same nude a few years back. you two share special bond in life, brother-sister relationship.
Dont drag any further, dont think much on this, assume that someone else also saw you nude but didnot give a crap. The same applies with you. You have nothing to do, thinking of other girls will not help in anyway, rather be that brother who is supposed to protect his sister from all evil, thats what will help you. Spent time with her, talk on study and others, that will ease things off, not hiding around and jerking off.