Leah's a tricky one, and can be more identified by what she doesn't do than what she does do. Her lack of unique language use actually sets her apart from her sisters. For example, Adah constantly uses plays on words and palindromes; palindromes are words that read the same whether read forwards or backwards (for example, "ma handed edna ham" as a sentence palindrome). Adah also uses rhymes, poetry, and other plays on words. Adah's narrative is tinged with cynicism, sarcasm and a bit of darkness also. So that sets her narrative apart. Ruth May is little, so of course her narrative is much simpler, she uses incorrect words because of her developing vocabulary, and has very child-like perceptions and wording. So it's easy to tell her narrative apart from the others for those reasons. Then, Rachel's narratives are full of sass, whining, and misused words. She says a lot of words so seriously, but she is completely misusing them; for example, in her very first passage, she says "executrate" when she means "execute".
So, what sets Leah's narrative apart is the absence of those unique identifiers that are very easily seen in her sisters' writing. Leah's narrative is very factual, informative, and filled with details that often the other sisters leave out. She focuses a lot on the people of Africa, and expresses a fascination and respect for them that doesn't come through in her sisters' perspective. Near the beginning of the book, watch for her adoration of her father that comes out also. She tends to use longer sentences with a lot of commas, and educated word choices to describe the situation. She has great sentence variety, starting her sentences differently quite often.
I hope that thought helps to get you started; good luck.