Looks like she is out growing naps. My son did around that time too. I used a playpen for those moments of near insanity when I needed a break. He loved it. A big square one did the trick.
He played in it and enjoyed the safety of it. Of course I did not leave him indefinitely and he knew it was a short break for both of us.
Once he was walking, I would leave half side down and he would crawl in there and just play or even nap if he was tired - all by himself. I think it made him feel safe - made his world a whole lot smaller and more manageable for him.
Many people do not believe in play pens - not sure why. Did not have one for my first but did for my second and it was wonderful.
You are not expected to be holding baby or tending to baby every minute of the day and this gives her a safe place to be while you are doing laundry or preparing dinner, playing with brother or doing your hair. What a treat it is to take care of ourselves. :)
As long as she can see you she will be great. Add a few different toys each time to keep her occupied.
Two babies, bless your heart. I am sure quite exhausting. Hang in there! C.