My son and sister

I am the mother of three teenaged boys and also have a recently divorced sister with two teenaged girls. There has always been frequent contact between our families, and my boys have always been regular visitors to my sister. A while ago, my two younger sons informed me that they suspect their older brother of having an affair with his aunt. I confronted both my sister and son about the allegations, but both denied any wrongdoing. After this, I forbid my children from visiting their aunt again, but have then discovered that the eldest still visits his aunt, which seem to lend credence to the story of them having an affair. About a week ago, I learnt that my sister is pregnant, and I am afraid that my son might be the father of my sister’s unborn baby. Whether my son has fathered my sister’s baby or not, I feel that I need to find a way of seperating the two of them. Any advice?