Look before you leap! Realise the ramifications of reporting your own flesh and blood to Social Services!
For who's to say that it wouldn't cause the complete breakdown of your relationship with your sister, and even your nieces as well?
Think seriously before you do anything you might regret for the rest of your days. You say "Not sure what to do." And that alone should make you think hard before doing anything rash.
Look at the facts as you've described them:
1. Your 14 year old niece is 4 months pregnant.
2. She smokes and drinks.
3. Her mother resents your "interference".
4. You are concerned about the younger sister as well.
You are considering telling Social Services. Why? To what end? Put yourself in your sister's place and ask how you would feel is you had SS land on your doorstep out of the blue because your 14yo daughter is preggers?
Now, I'm not condoning the fact that this teenage girl is pregnant, but this is far from rare, and will happen a zillion times in the future throughout the world.
She smokes and has had alcohol. Hands up how many adults did not do that as a teenager. Not too many hands up, I see?
You suspect the younger girl "may follow in her footsteps shortly." What leads you to believe that?
By contacting SS will mean that your sister could, possibly, lose at least one of her children. It also means that the family may then be placed on an "at risk register", there will be home visits, total strangers will be dictating to your sister how she must act as a parent, your niece will have her "own" Social Worker", and every man and his dog in the neighbourhood will know.
Do you seriously believe that your sister is a totally irresponsible mother and that your nieces are at significant risk as a result?
You must come to terms with these possibilities before making