Both Princesses can attend Royal Ascot if they want, along with their husbands. The 5 day Race Meeting is open to everyone. You pay for a day ticket and attend. What this is all about is whether they and their husbands are allowed to be in the Royal Enclosure and take part in the horse carriage procession. I, personally, don't think they would want to embarrass their Uncle and Cousins and will decline an invite to be in the Royal Enclosure and Carriage ride. There is nothing, however, preventing them attending the Race Meeting.

https://www.gbnews.com/royal/king-charles-olive-branch-princess-beatrice-princess-eugenie-ascot-ban