My oldest child is from a relationship I stupidly got involved with at 18 - with a then-19 year old who was hideously abusive in all senses (I had to pay his supposed rent to his parents when I got my first job, because he was unemployed... he verbally abused me, telling me that I was "thick" purely because I hadn't taken a language for GCSE... he was physically and sexually abusive...). I literally ran from him, two months before I discovered that I was pregnant with my daughter - who considers my ex as her father, seeing as she's known him her entire life. Her biological father has never been involved in her life.
However, I do know that he's now married with three other children.
I know this because his wife and her sister did to me, and my daughter, exactly what you have done to your partner's ex-wife and daughters. My daughter - who knows a little of what went on - and I have both been left feeling absolutely violated. It feels, for me, as though he stepped into the life I have sacrificed and striven to build for my daughter and myself, and... chipped away at its foundations in some way. Even though, to the best of my knowledge, he knew nothing about his wife and sister-in-law's actions. For my daughter to discover that she has younger half-siblings, other than her adored little brother...? Almost crippled her. She still asks, from time to time, if they're all being abused, too, and maybe that's why they sought us out on social media.
I don't think that they are. I hope that they're not. I hope that he changed, that he grew up, that he realised why I ran and why he wasn't allowed within a 10 foot radius of my precious baby. He tormented me throughout the pregnancy, and then his mother made the first few months of my daughter's life an absolute misery. My daughter had a relationship with her biologically paternal grandparents for a few years until she (my daughter) also chose to run from them. I don't, to this day, know why. She claims not to, either, but as she has MH issues which no one understands the trigger for... perhaps they did something to her. They stalked her relentlessly through college, which didn't help.
Leave this well-alone. But be aware that there will be a reason as to why your partner doesn't have a relationship with his oldest daughters. And you might not like to actually discover what that reason is...