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.