I'm not doubting your devotion to your sister. And, in case you've forgotten, she's part of my family too. My concern in that arrangement is for Alexander. I remember how you were with the twins when they were infants. Alex is not going to grow out of this phase for quite some time.
Just... If things get too intense with having a baby in the house along with everything else, let them come here for a few days until things simmer down. Is that acceptable?