"My mother has a goat and a donkey."
Translation:Shimá tłʼízí dóó télii bee hólǫ́.
Bí does exist in Navajo, although it hasn't been introduced in this course. Wiktionary says it is a third-person singular or dual pronoun or a possessive pronoun. (Wiktionary has a nice example sentence, "Łį́į́łgaii éí shicheii bí," meaning "The white horse is my grandfather's."
Bee, on the other hand, is a postposition meaning "with, by means of, or by means of it," and is used in this case to mark the things that she has. Beginners beware that the sounds ii and ee are NOT the same, even if they are in the same tone.