Living wills

I have a question about wills so my step dad was diagnosed with lung cancer back in Oct of 2024 it shrunk but from January 2025 to now his cancer grew back. Unfortunately he doesn’t have a will yet he is married to my mom for about 16 years now and has 1 daughter who he barely sees and 2 grandchildren from her. They live far away. I have been caring for my mom and step dad the whole time basically as I lived w them most of the time recently bought a house w my husband 3 years ago but him and my mom helped me raise my 15 year old son so he is super close to him and myself and I have a sister who has 3 children none close to him like my son just a little concept of our family. Anyway I want to help get his affairs in order as I don’t want any drama with the family. I don’t believe my sister, son or myself gets anything from him (we wouldn’t want anything anyway) unless he writes that he wants us to have such and such in his will right?! Bc I know he wants to leave my son his jet ski they bonded over that. Him and my son rode jet skis together since my son was a baby. I was also wondering my mom has been his house wife the whole time as he made good money and worked for the union and works as a engineer operator w the big yellow diggers they recently refinanced the house w his name on the mortgage w hers of course. My thing is as tough as a situation this is and so ugly when someone is sick to make them write a will but I feel it’s in his best interest so no one gets screwed mainly between his daughter and my mom. I don’t know what she gets and what his daughter gets if it’s 50% 50% if no will is written up or if it all goes to my mom his wife and then she leaves everything to my sister, step sister and me or how does something like this work. My mom always kept a clean house home cooked meals etc and hasn’t worked so I don’t want this to get sloppy. It’s so hard to talk about this when someone is so sick but idk what to do and being closest to my mom and step dad I feel I need to help align the affairs. Can someone please walk me through on what to do how to do it explain who gets what here and if I can write the will and have him sign in front of 2 adult witnesses and then sign I do think I know that much we are in Nj for reference. Please help!