How to guide a teenager who isn't my son (salam)

Salam. For context, my wife and I have been married five years and her younger brother, 16, moved in with us after their last parent passed away. He struggles with personal hygiene and tidiness and generally behaves immaturely - probably because he grew up without a consistent father figure. He leaves urine on the toilet seat, doesn’t wash himself properly, and drops dirty, stained underwear around the house. I’ve tried being kind, patient and gently instructive, but it keeps being ignored. When I bring it up to my wife (his sister) I keep details vague to avoid embarrassing him, but she’s said that if he needs correcting it’s my responsibility. She’s the legal guardian now and wants me to become a guardian as well, which is in process. She’s told me to do what’s necessary, but he won’t listen and I end up cleaning up after him. I taught him basic washing practices, how to shave impure hairs, and to put dirty clothes in the laundry basket in the hallway, but none of that stuck. I understand a lot of this comes from missing a father, and I’ve tried to be gentle, but he seems not to care. When I show any frustration he snaps, “You aren’t my dad, so stop telling me what to do.” I’m at a loss about how to be firm yet compassionate - he just pushes back. How can I teach and guide him effectively? I’ve tried kindness and patience; what should I try next? JazakAllah khair.