Cherry80, I don't have any personal experience in this area, but one of my BFs has had a very similar experience.
She met her now-DH about 9 years ago. He had left his DW and three children (DDs 14 and 17 and DS 19) and met my BF very soon afterwards, too soon some people thought, but like you, they weren't together before the marriage break-up.
The Ex-W was desperately upset and made my BF and her DP's lives hell through her DDs. She would tell them nasty stories about their Dad and his new GF and they stopped seeing him. However, his DS took to my friend straight away and they became very good friends, eventually the DS moved in with my friend and his Dad.
After many years, things started to mellow and my BF and her DP were married this year. Both DDs were at the wedding and read out poems at the service. My BF says that she cannot believe how miserable they'd made her for so long because now that they've grown up, they're so very fond of her. They're both working now and appear to have grown up since leaving their Mum's clutches.
So all I am really saying, is hang in there. It's not going to happen quickly and may not happen until the girls have left home. But don't let that destroy your time with your new baby. My BF wasn't so lucky as her DH had already had the snip after having his 3 children, so now, at 40, it's unlikely that my friend will ever have a child of her own. But she made the choice to marry her DH and they are very happy.
Things will be tough, but you will all get through it. She's clearly a woman scorned!