I think you, Vince, your mom, and all else, could use a bit of counseling. Look into it. Family and friends often want to believe so hard that a few months of sobriety means that the addict is "cured," but the truth is it can take years, and even then, it's a constant struggle. Think of sobriety as brain cancer. It can seem to go away, but the word "cured" won't be used. It's remission. It can come back. A drug habit can come back. It's constant vigilance. She has to keep working on it. People in 12-step programs don't leave at the end of the course. They keep going, and proudly stand to declare it's been X years, Y months Z days.
I've known more than my fair share of addicts. Most of my relatives are addicts. My mom is an alcohol addict. My dad was one until he shot himself. The only ones who've stayed sober for loner than a couple years are those very few who at least monthly go to meetings to keep working on it.
There is a red flag I see here. Your sister is choosing as friends people who are fellow addicts, and she very likely feels that they understand her more than anyone else. Their common bond is drugs. She's no longer distancing herself as much as possible from her extensive drug-history. She's bringing it right back into her life.
Those friends might be good people, but she needs right now to be as far from anything related to drugs as possible.
Is Vince willing to put up with this for many years to come? He may be in love with her, with who she is as a person, but her vice may still be too much for him to handle for years without having support himself.
Another red flag is that, when he asked if she still loved him, after she accepted an engagement ring, she said she couldn't answer. If she did, it would have been quicker to reply with three letters.