What should I do if I find my sister having an affair with my boyfriend?

It depends on your boyfriend. If he has an ambiguous attitude towards your sister, leave him at once. If he dares to be tempted by your sister now, can you guarantee that he will not be tempted by other women in the future? If you don't let go when it's time to let go, you will be put in an awkward position.

If not, talk to your sister. Tell her not to touch her heart.