How can I make my girlfriend feel better ?

My girlfriend is pregnant with her second baby she has a son from her last marriage he's almost 11 she's worried that him and his new brother or sister won't get along well. Her step brother is 12 years older than her they don't hardly ever see each other he hasn't been around in years. She's worried she's made a mistake she also feels different with this baby at 28 than what she did with her son at 17 she's starting to feel like this was a bad idea. How can I help her feel better ?