OP: Is the Sinhalese language very different to Pali?
No, Sinhalese language has many similarities to Pali. Many of the words in Sinhalese language were derived from Pali and Sanskrit.
In the introduction chapter of the book, "An Introduction to Kachchayana's Grammar of the Pali Language" by James D'Alwis, it has stated that;
THERE is hardly a country on the face of the Globe which presents greater facilities for acquiring a knowledge of the Pali, than Ceylon; and, perhaps, no nation possesses greater advantages for its study than Sinhalese. Pali, like the Sanskrit and the Sinhalese, forms a necessary part of the course of education pursued by the natives. Our Alphabet is common to these several languages, and the affinity which the Pali bears to the Sinhalese, both verbally and gramatically, renders its study far more easy to the people of this country than even to the Burmese.
Although the Sinhalese, as a language, has been latterly neglected; the Pali, from its being the dialect in which the Buddhist scriptures are recorded, has always been the principal study of the largest portion of the Ceylonese, who are followers of Buddha.
~ Cited from: An Introduction to Kachchayana's Grammar
OP: Should Sinhalese be able to learn Pali easier than Westerners?
The answer should be yes to this question according to the above quoted passage mentioned in the answer to the first part of the question.