I may become a father at 19 years old?

I am a faithful boyfriend of a 17 year old with a very rough home life with her mother and her step father. She is the care taker of a 1 year old a 3 year old and a 5 year old because her mother finds it as free baby sitting. She may be pregnant showing all the signs of early pregnancy and having bleeding 14 days into her cycle. I have a home life in an upper middle class family and i work a very good part time job bringing in around 400 dollars every two weeks. The problem will not come from my side as i have very loving and compassionate parents whom will not see me suffer or my child suffer but the problem will lie in telling her hostile mother the fact that i impregnated her daughter. Her mother was pregnant with her at the age of 18 and her mother was pregnant at the same time. How can i go about letting them in on the situation without having a huge blowout with her mother while at the same time protecting the love of my life and my child from physical and emotional harm from her mother. I know there isn't an easy way to go about this but any experience help will be much appreciated as I am trying to be the strong one for her and take care of the financial part of her pregnancy and the emotional part.