Nancy Springer 
Great Pony Hassle [EPUB ebook] 

4 girls and only 1 pony! When their mother remarries, twin sisters Toni and Staci get . . . twin stepsisters! Toni and Staci have almost nothing in common with their new siblings Paisley is loud and spoiled, and Stirling is pretty and perfectbut all of the girls love horses. When Paisley brings home a pony of her own, with a sugar-colored mane and sweet dark eyes, the rest of the sisters are consumed with jealousy. How can 4 horse-crazy girls get along when there’s only 1 pony?