If it's been 6 years and no marriage and your relationship is strong then why want to marry now?
What has kept you from marrying her up until now? Why now? And how do you get along with the kids?
Those are the questions I would be asking myself if I were having to make this decision.
Marriage is a big BIG commitment. Unfortunately in today's world, most people take it for granted and it has lost it's meaning along with everything else. Marriage is more than an economic decision. Marriage is more than a religious rite. Marriage is something that, if you ever hope it to work, must mean the same thing to both people. If you are undecided, express that to her......it is far more dangerous to give into the commitment when both parties aren't giving their entire selves to it. Personally, marriage doesn't mean the same thing today that it meant to our parents, or grand parents. Today it is just a word, like love, tossed around with no expectations of consequence for the weight and meaning that such a word has......if you are not sure, wait. Love can exist without marriage, just as marriage can exist without love.
I would recommend sitting down together with your loved one and writing all the pros and cons to marriage.....think it through...if the pros outweigh the cons, get married, if not then don't.....that easy. Good luck!!!!
I'll assume the daughters are old enough to take care of themselves given your age. If not, then think thoroughly before taking in kid into your life because they will become your responsibility too.
If you truly love her and the feeling is mutual, then go get married. True love is rare!