Firstly, she's a married woman with a husband and two kids, it's about time that they stood on their own two feet and stopped relying on dad (and step-mum) to bail them out! Does your DH have some kind of guilt feelings over the break-up of his previous marriage, that he feels he has to over compensate, and try to make things up to his daughter with all the financial assistance? To be honest, giving financial help to our adult children, when it's really needed, is one thing, but this seems a bit way over the top with houses, businesses and cars, etc.!
Secondly, it's up to you and your DH how you decide to celebrate his 70th, and you both chose a holiday, which sounds perfect! Months ago, you tried to arrange a family gathering to please his daughter, and she ignored your efforts. Now, closer to the birthday, she's apparently 'desperate' to arrange something for her dad. This all seems very deliberate, probably to spite you and your efforts.
I say, just let her do it! We all know that arranging gatherings, either with family or friends, can often cause stress and headaches with the logistics of getting everyone available at the same time, and making dinner reservations, and trying to please everyone, etc. If I were you, I'd just sit back and leave the arrangements up to her, saving yourself the bother!