I wrote a bunch on more concrete tools and then lost all of it. So briefly.
Don't give away your power because you think the exwife could get revenge by manipulating your stepson. If he's already warmed up to you as stepmom it means one or both of 2 things: 1. he likes you a lot, and 2. he's aware that his mom isn't very stable or possibly even very safe for him. You and his father may already represent so much more security to him that he may let you know whenever she does things he doesn't like. And if alcohol plays a big role, you could have the tools to get better custody.
Her bullying your husband is very disturbing, but bullying her son is certifiable child abuse. You can use art therapy methods to find out if he's aware of a fearful relationship with her: Find a way to get him to write a story about a boy living alone with his mother in the forest--fairy-tale style. A lot depends on his age, but if it's obviously fiction, he may not realize how much he's revealing how he really reacts to his mother.
Don't hold back from confronting his mother because of what she MIGHT do behind your back, but strengthen your stepson's trust in you by finding out if he feels pushed around by his mother, or reacts to moodiness or personality changes when she's drinking. He'll need understanding from someone he can trust to cope with her, if she's not controlling herself when alone with him, and you can spend quality time with him to gain that trust and offer that understanding. It's really rather unusual for a stepmother to get accepted wholeheartedly as quickly as you have been (apparently) so you might be able to strip her of that power you're afraid she might wield, just by really being the good mother you want to be.
Many an angry exwife has accused her exhusband of molesting their daughter to hurt him and win things in court. But there are handles on either end of that stick, and in this case SHE is more vulnerable to child-abuse investigation, if she's bullying him or making him very uncomfortable and he doesn't want to be stuck with her as much as he is now. So to the degree that you gain the boy's increasing trust, you can turn the tables on her, and you don't have to let your fiance's habitual passivity in the face of his exwife's bullying put you in the back seat of the family car. You can eventually show HIM how to stand up to her by being the good mother to the boy that she probably can't be, and then pushing her back as you have begun to do already.
I may be oversimplifying. But if the son is the weakest link, then form your good parental alliance with him by giving him the good acts of love you want him to grow up with.