If your cats are different instars, it's quite likely that they are not siblings. The eggs would have been laid several days apart, and I think Monarchs tend to move from place to place when they are ovipositing--though I never actually followed one around LOL. So, if you're lucky enough to have a male and a female, you could have a budding romance. But, chances are there will be more Monarchs coming your way soon anyway. So, hopefully you'll have more eggs and cats on your milkweed before you know it.

Martha

Terrene, interesting question! If I remember correctly I think the answer is yes, and I also think the butterflies are potentially less healthy if they do. I'll let a resident expert chime in on that though.

From what I've witnessed in my years raising monarchs is that they are generally gone from the area within a day.

However, last year from the end of July until mid september we had monarchs living in our yard. I saw mating, wild chases around the house, and 3 monarchs even died in our yard. How come? Two words: Liatris Ligulistylis