Sibling advice

I have a baby due next summer with my partner and have a 6 year old from  my marriage to my ex husband. It has typically happened that the baby is due on the 2 weeks of the year I could have done with avoiding as my daughter is due to be away on holiday with her dad. I am constantly worrying about the implications to her in the long run of her not being around when the baby is born or worse still that the baby arrives and she then gets whisked away. I am going to speak to my ex about it when I have had my scan and can tell people but I just wondered if anyone has been in this situation and what is the best way to deal with it. I want to ask him to not take her just on this one time (we are amicable and share the time and holiday we have with her) which I know will cause problems but any other time would be fine, I just feel she should be around. Any thoughts? I want to enjoy my pregnancy but feel like this is looming over me the whole time and I am panicking about it.