Found book!

[telophase]

I believe the second book that I was looking for in this post has now been found! My Sister the Horse by Barbara Klimowicz. The Amazon.com description reads, "The adventures of ten-year-old Alice Polaski, who lives in an old building with condemned balconies and whose five-year-old sister thinks she's a horse," which sounds a lot like it. Now to see if I can get my hands on a copy...