Help me with my manga's story line (Is step cousin incest--)

So I wanna draw a coming of age story about this girl going to her grandmas over summer and blah blah blah- anyways she meets a boy there and..... Like will it be bad if that is her new step cousin she hasn't met and will fall for him? Or will that be gross and replace him with something else... Kinda wanna make it as a forbidden love typa thing but not too wrong either....