WIBTA to say no to sharing family Christmas

I (F42) and my sister (F39) are the only two children of our mother. Our father died 7 years ago. My sister (let’s call her Mary) has two sons and a husband. I have one son and am divorced. Mary offered to host Christmas this year and that was fine until today when she said she plans to invite 3 (male) “orphans” from her workplace to join us. I don’t want this to happen because:

1. We are only 5 adults (me, Mary, her husband, our mother and our elderly uncle) and 3 kids. Adding 3 strangers to an already very small family group will tip the balance and make it an awkward day.
2. My son (10) has autism and will really struggle to understand and cope with 3 random male strangers at our Christmas celebration.
3. I always strive to have family traditions (which admittedly are hard to keep up in such a small family), but adding 3 (to us) random strangers will really affect the dynamics. I also think the 3 workmates probably wouldn’t want to come if they understood how intimate our family group is. They aren’t really orphans after all (they could do something the 3 of them and almost be the same sized group as us!). My sister has always wanted to invite “orphans” and when it was just one genuine, hard done by loner, I welcomed them too. But 3 workmates when we are only 5 adults in the first place is too much. AITAH to kick up a fuss and say no?