Hypothetical situation

Let's say you have reason to think your sister killed her baby daddy because she has an unhealthy obsession with her son and wants her kids all to herself. What would you do? Son is an adult now hypothetically. But still lives at home because his mom has convinced and manipulated him into thinking no one will love him

Edit: Hypothetically speaking, lets say he has a necklace that said best dad, hypothetically son gave it to him for father's day. Wore it everyday. When body was found, he wasn't wearing. Hypothetically speaking you with your nephew to have a talk with him about this girlfriend he has that my sister hates because she's the only one who can love him. Hypothetically when sister comes back you cut the Convo short and you go to the bathroom before you leave and hypothetically the only working one is in her bedroom at the moment. Hypothetically you notice the pair of earrings she borrowed but never gave back on her dresser, you get mad and take them and hypothetically you see in her box another pair of jewelry that was never returned and then hypothetically... Find that necklace. Then hypothetically you go on Facebook. And find her baby daddy's Facebook. Scroll years down to his last post before he died and he was wearing that necklace. Hypothetically....