Life and its complications

Suddenly you discover you are at a marriageable age, friends are getting married around you and it seems like you are going no where near that as you are breaking up a relationship everyone thouht was def going down the aisle. One thing i have learnt, try (its hard tho) not to introduce the guy to the world until you are very sure but then does that make a difference? We have heard of relationships that broke even at the altar so i think its just God and it happening when it is to happen.
Take my sis for instance, she's almost thirty, she's pretty, so so so so nice (nicer than me i admit), not given to problems and issues, gentle, educated (has MBA Finance) and all the walks, u know. Not cos she's my sis but she's just your typical take home to Mama. Unfortunately,no guy has deemed it fit to do that instead they all have broken her heart after coming as far as introducing her to folks, meeting her and all then they just leave either cos of an old girlfriend or some equally lame excuse like not agreeing on having a big or small wedding (can you beat that?).
Anyways, while all these was going on, there Mr. Not Really Married(NRM) but lives with a woman who is a bad woman and whom he does not love and they have two kids together hanging around encouraging her and holding her hand each time there is a break up say for like 5 yrs now and say he loves her. Now this is where the problem is, my sister now thinks she should marry this Mr, NRM! For God sakes he is still with this woman and they have two kids, he says he will leave her and come live with my sis and that he has told d woman who initially didn't agree and then says she will agree on one condition, Mr. NRM is to continue to take care of her and the kids (please!)
I have spoken with my sis, she says she does not know how to tell him and that no one has been this crazy about her bla bla bla, please help me save her, what do i do to let her know that she'll always be second fiddle,that she won't get the satisfaction of marriage here, that she is going to be in a polygamous situation which we are born into and have sworn not to enter, Help me blogsville!