Undeniably Yours by Shannon Stacey

Image linked from Goodreads 

We met Kevin Kowalski in Exclusively Yours as the brother of the hero. In this book, he gets his own HEA. Beth Hansen is a floater. She moves through life on her own terms, not setting down roots anywhere. Which is why it is a shock to come up pregnant after a one-night stand where protection was used. A one-night stand where she believed he was a player.
Kevin immediately jumps into protective mode. He just wants to make sure that Beth is taken care of. None of her jobs provide insurance and her apartment has... issues. So she moves in across from Kevin.
I loved that this story took place over a year. Kevin learned from his brother's mistake. Don't marry just because of the baby. Insurance wouldn't cover it anyway. But he thinks he's falling in love. It's sort of insta-love but we actually get to see that it sticks around.