My mother has Alzheimer's and is in a convalescent home my sister and I dont get along..can she prevent me from visiting my mother .

Your mother, not your sister, has the right to decide who visits.  Even if your sister has guardianship of your mother's person, she can only limit visitors to exclude those who have been abusive or exploitive.  If you get to the point of needing a demand letter from an elder lawyer, you might use the Find a Lawyer function on the website of the National Academy of Elder Law Attorneys (www.naela.org)