Hi Ofelia,
You and YOUR SON are your priority. Your house is very toxic right now. And you have chosen to deal with your pain, issues, frustration, resentment, anger with alcohol. This is very dangerous for you. Ofelia, I want you to live a healthy life for you and your son.
1st, your name is on the title so you have a LEGAL right to say what happens in that house.
2nd, talk to your significant other alone. Let him know exactly how you are feeling...disrespected since his adult children bring their girlfriends over, they don't work consistently, nor do they contribute to the household via chores or financially. Tell him that you both need to talk to then and you need your significant others TOTAL SUPPORT AND BACKUP. He needs to lay the law down with his sons. If he can't do that in any way, you need to be prepared to leave with your son and take your home equity with you.
3rd, you don't want or need negative influences around your son.
Ofelia, you just don't marry your significant other, you marry his family. If this is any evidence of what your life is going to be like when you marry, it will only get worse if he (your fiance) can't put his foot down and practice "tough love". You are young, you are only 10 years older than your fiance's eldest son, girl, go live your life and find another man that will love you and your son unconditionally.
Ofelia, your heart is in this, but your significant other hasn't stepped up to the plate to marry you. Sounds like his sons lack the same committment with their work ethic. The apple doesn't fall far from the tree. So don't eat the fruit and put down the bottle!
If you choose to stay, I agree with Madeline M, you and your fiance should draw up a contract that CLEARLY states the conditions of living in the home (working, contributing financially, no girlfriends spending the night, etc). If your fiance and/or his sons can't handle this, then it's time for all three of them to go or pack your bags and hit the road with your son.