Sullivan. It's a proper name. Of Irish origin. You would not change it: it IS the person's name!
To avoid ambiguity, if you wanted a Spanish-speaker to say it as in English, you could spell it with one 'l' - 'Sulivan', but even then it would emerge as something like 'Sooliban'.
The letters 'll' in Spanish are pronounced a bit like 'y' as in English ''yet'; so a Spanish-speaker encountering (written) 'Sullivan' for the first time, might pronounce it something like 'Soolyiban' ('v' in Spanish approximates to 'b').
La casa de Sullivan.
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Sister in law in Spanish is cunada.
How to say "hi" in spanish is Hola. How to say "bye" in spanish is Adios.
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To say stepfather in Spanish, you say padrastro.