First, let me say that my husband and I really appreciate the thought that went into sending flowers for my birthday. They were beautiful. Thank you.
That said, I would prefer that you not send flowers. Some of the flowers were lilies, which are toxic to cats. Also, my cats have a tendency to knock over the vases, which makes a mess and wastes the beautiful flowers.
Rather than flowers, consider sending ...
Or what I would really like would be something like ...
Again, we appreciate the thought. Thank you for taking the time to send us a gift.
The first ... should be replaced with something of similar expense to flowers, possibly something transient like candy or something like books or artwork. Ideally something that can also be delivered. Perhaps a singing telegram or a fruit basket. If this is an actual gift that you've received in the past, that's even better.
You can go wider on the second .... Perhaps something more permanent like cutlery or a pan. You might even point them at your wishlist on Amazon or a similar retailer. And when your husband's birthday is coming up, you might send out his wishlist.
This communicates how you appreciate the gift and most especially the thought behind it. It then moves to explain why you didn't appreciate the gift as much as you might have. It offers alternatives, so in the future, they can give you gifts that you can appreciate more fully. Finally, it repeats the appreciation, so it doesn't get lost in the criticism.
To seal the deal, keep your eye out for opportunities to send them gifts, quite possibly flowers. One reason why they might be sending them is that they like them as a gift.
It might even help to try to communicate with your siblings to arrange a similar group gift of flowers to another sibling who is having a birthday or similar celebration. If they say, "But I thought that you didn't like flowers," you can point out that you don't like receiving flowers but you are fine with giving them. And presumably the recipient doesn't have the same issue with pets.
I'm ignoring the possibility of just thanking them for the flowers. Advising you what to do is ranging off-topic and really depends on the details of your situation. My only real thought is that thanking them without further feedback may result in more deliveries in the future. The easiest time to say that flowers are inappropriate is the first time. If it becomes a thing, it only gets harder.