Hello! Please remember that my response is for information only, we are not establishing a therapeutic relationship.
I want to clarify a few things ---she already kicked the boyfriend out of the house and took his keys, but you are still concerned? Are you concerned they will get back together?
And the trainer --that's a different man, right? He hit on her and she didn't fire him--but this is of concern to you? What concern do you have about the trainer?
It's got to be incredibly frustrating to watch your sister hang onto this man. From what you've said, it sounds like a terrible situation.
Here's some thoughts on what you might say: "I am worried about you because I see him trying to control you in this way: (list a, b, c)." or "I am concerned for (name of daughter) because she did nothing wrong (posting pics on FB), but yet he yelled and is punishing her for that. That's going to make her feel bad, confused and guilty. Is that how you want her to feel?"
If you notice, I tried to stick with "I" statements rather than "Why? (why do you stay with a guy like this) to try to keep her from getting defensive. Also, try to keep it as factual and objective as possible meaning "He hasn't come home for 2 weeks," (fact) rather than just subjective characteristics "He's a lying jerk."
When you talk to her, your tone is just as important as the content --even though you have a lot of emotion about this, you want to be as calm and objective as possible. What you are really trying to do is hold a "mirror" up to her so she can see for herself. Mirrors don't yell or give their own opinion, they just reflect what is there.
If you want to give me some more examples I could write some more statements.
Sometimes it's hardest for us to hear from those that are closest to us --so it's true that she may not be able to hear you even if you say everything "perfectly."
The pic posting was NOT your error, and for you all to comply with that is to "enable" him to keep his double life. It's also telling your niece that she's responsible for "covering up" for his lies. I wonder if his fiancee would stay with him if she knew about your sister.
Anyway --you are not giving her advice, you are merely stating facts --what you see. If she mentions your divorce, just agree with her, "Yes, I made mistakes," or "Yes, I picked a loser," (or whatever the case may be). "I am not giving you advice, I am merely repeating what he has done (or not done) so far."
You don't have to use the words, "If he wanted to commit he would have," Instead, use the words, "It's been X years, and he hasn't committed to you."
Do you see the difference? With the first statement you are making a judgment or prediction. With the second statement you are just stating the facts.
If she doesn't listen, you may need to consider dropping it. I think I'd avoid "you deserve better," not that it's not true, but because she's not likely to hear it. "It sounds like a life with him means he's gone X days out of the week to be with her (fiancee). Are you OK with that?" If she says, "He's going to leave her," say, "OK. I wasn't sure since it's been X years." (keep going back to the mirror).
You can give me more examples if you'd like.