A 13-bedroom estate on gated La Gorce Island, for sale for $10.8 million, just housed a special guest, Rosemarie Kennedy, 82, younger sister of President John F. Kennedy. She's the aunt of Miami Beach resident Anthony Shriver, 35, founder of the Best Buddies. The house rents for $39,000 a month furnished, or $55,000 a month "seasonal", says a Wimbish-Riteway flier from Jill Eber and Jill Hertzberg. Shriver says his mentally disabled aunt came here with caretaker nuns from a Wisconsin convent. Here she attended Mass at St. Patrick Church, visited Viscaya and Parrot Jungle, and swam in the pool daily. "Strong as an ox," says Shriver. Amongst her visitors: brother Ted Kennedy and wife Victoria; sisters Jean Kennedy Smith and Pat Kennedy Lawford, and Anthony's folks'.