Advice needed on domestic violence situation

I know there are lots of amazing people here with both personal and professional experiences in dealing with domestic violence and so I’d like to ask your advice on a delicate and very serious family situation.

In a nutshell, my younger sister M, who is 22, has been dating a guy I’ll call Bob for the past two years.  M has always been overweight and struggled with self-esteem issues and Bob is her first boyfriend.  Shortly after the relationship began, our family noticed M begin pulling away from us and we attributed it to the throes of young love.  As the isolation increased however, we had several sit-down talks with her to share our concerns, but none seemed to be effective.

Bob and M’s relationship has been fiery from the start.  They argue – A LOT – and get into fights that sometimes turn physical on both sides.   In the beginning, it was minor stuff like pushing or shoving, but as I’ve since discovered, it has escalated into more serious abuse.  At one point last year, M called my mom in tears because Bob was driving her car recklessly and wouldn’t let her out.  My mom called the State Highway Patrol and later had M talk to a few police officer friends about domestic violence is.  M listened, but refused to admit that Bob’s abuse qualified as domestic violence.

M calls my mom whenever she and Bob fight, but then later lies about
the fights and/or claims they are her fault.  Bob has M convinced that
the fights are all her fault because she has been diagnosed with
bi-polar disorder.  She also tells different stories to my mom, my
brother and to me, so we never know which version is true.  My mom has
gone so far as to threaten Bob face-to-face, but Bob appears to be a
skillful abuser.  He tells M that no one will ever love her and has
slowly gained control over virtually every facet of her life, including
her finances.  M joined his church, which is very conservative and
preaches that women should obey their husbands, wear only skirts or
dresses and not cut or color their hair or wear jewelry. The
relationship is not sexual, but he lives with M and my dad (who is a
useless, selfish jerk who won’t kick Bob out of the house like he ought
to).

My brother and mom are highly (and rightly) judgmental of
Bob but that only makes M feel as if she can’t go to them for help. 
I’ve known about some of the problems between M and Bob and my way of
dealing with it is to appear non-judgmental so that she will feel free
to confide in me, which she does… sometimes and mostly with lies and
half-truths.  I try to get her to look at my own marriage and that of
my other brother, both of which have never seen any yelling let alone
physical violence, as a way to get her to see how dysfunctional her
relationship with Bob really is, but she insists that she loves him and
wants no one else but him.

I recently found out about an
incident that concerns me and my family greatly.  In September, M and
Bob got in another fight and M locked herself in her room.  Bob
literally kicked in the door and got M in a chokehold and insinuated
something like he could cut off her airway if he wanted to.  M ordered
Bob out of the house and somehow the police were called.  M told the
police about the chokehold, but Bob denied it.  Although the doorframe
was broken, the police said that because Bob lives there, they couldn’t
do anything about it.  Bob told police that M slapped him, which may be
true.  The police said that they could see a mark from the slap, so
they ARRESTED MY SISTER for domestic violence!  She
was HANDCUFFED and had to SPEND A NIGHT at the county jail.  My mother
had to see her youngest child brought out in handcuffs and leg shackles
and dressed in an orange jumpsuit before she posted a $2,000 bond for
M, which mandated that she could have no contact with Bob.

The charges were later reduced to menacing, but M still has a
misdemeanor record and has to go to court-ordered counseling and anger
management classes.  My mom only recently found out that within a week
after getting out of jail, M and Bob were back together despite the
restraining order.  That order has been dropped and Bob is once again
living with M.  As if the situation couldn’t get worse, I have now
learned that M has purchased a GUN for Bob, who is 20 and intends to enter the police academy in a few months. 
My family is divided on how to handle the situation.  Now that there
is a gun involved, I want to call an intervention and enact a
completely tough love kind of stance in which we do not allow Bob to
come to any family function and we give M ultimatums unless he at least
moves out of her house.  My mom and brother feel as if that would only
push her closer to Bob and further away from us.  I understand their
position, but this is the position we’ve all basically taken for the
past two years and not only has it not worked, the violence has only
escalated.

My dilemma is this: How can my family convince M that the next time might not be a chokehold but rather a gunshot to the head?