Taking your step son with you is extremely important.
I think the issue is more that until now he's been excluded from family holidays abroad. That's unforgivable in my view.
All your children are equal. You married your husband, knowing he has a son, therefore you are also 'marrying' the responsibility for that son, adopting him as your own flesh and blood.
The son has a half sister, he is a blood relation to your daughter, but even if he wasn't and your daughter had a different father, you are a family.
Children must must must be treated equally. Even if it's not what you would always choose, even if it might disadvantage your daughter, both your children have equal rights to your love, time and finances.
This may impact on your ability to afford things for your daughter. It may result in you only being able to go abroad ever other year, but this is the right thing to do.
The fall out from treating siblings differently can leave a lifetime of mental health issues. Please don't ever do this to either your daughter or your son.