I do however like the idea of replacing them with laxatives
That depends on how many toilets there are in the house. Unless you have more than two you don't want to risk them being out of action (or uninhabitable!) at the same times that no-one else can have a wee or anything.
I think they are both behaving appallingly - there is the theft (which may or may not be your DH this time, but has been in times past and he is unrepentant), there is the fact that these chocolates were particularly nice ones, and a gift , and thirdly, your DD2 was really looking forward to having one, and found that the box had been raided - effectively spoiled for her. Stealing a gift seems particularly spiteful to me. I would have cried, too.
DD2 will realise that the only way to keep her own sweets is to eat them all at once - not a lesson you want her to learn, nor is it something she wants to do. She isn't greedy, has the self-control to delay gratification (a brilliant life-lesson, btw) and dis effectively being punished for her virtues.
I agree the chocolates need to be replaced. Sadly there is probably nothing you can do to force your DH to contribute, other than going through his pockets when he is asleep, but you can tell your DD1 that she will be paying her own horse-related expenses for at least a fortnight. This is likely to be more than the value of the chocolates, but it will also teach her not to steal, lie or bully her sister (because this is effectively bullying). She needs to learn respect for other people's feelings and property.
Also, when DD1's and Dh's birthdays come along - buy a box of chocolates for DD2, instead of a gift for them. (And invest in a locking cupboard for DD2 - she needs to know that her things are safe when she leaves the house.)