Two things stand out, the first being that she sounds vulnerable, she is only 21 and so young, and she finds herself pregnant at 18 and instead of going out and having fun, she finds herself fast tracked into motherhood and adulthood. She has probably lost her friends, and on some levels she must be finding this very very hard.
Along comes you, a man that likes her, is older - she has nothing to lose she feels she may as well, as after all she is living a middle aged life already in some ways. But her youth has been lost, the time she should be partying, having fun and finding out who she is - it is all been eclipsed by an early pregnancy and now potentially an old man. And make no mistake you are old when you compare your ages.
I think it would be very unkind to take advantage of her in this way. I do think you will be heavily judged for it by everyone around you, and for as long as you are together you will be the seen as a sleaze.
Your soon to be teen dds will be mortified, and your ex will think you are having a mid life crisis.
Allow this young girl to find happiness and fun with someone who can really offer her a proper life, you are at best - have another ten years before the aches and pains and tiredness kicks in.
You should tell you think she is amazing, and she deserves someone that can give her the fun and excitement that she deserves, she doesn't need another Dad in her life.