The fact that you were searching for Laura Louise in 1911 made me think you already had her on the 1891 and 1901 censuses, but on checking back I had difficulty locating her on either of those Is she the Louisa that appears in Brighton on the 1891 and where was she in 1901?
If you have not got her on the 1901 either, could she be the Laura Ellen Grace aged 18 that died in the Horsham district in 1899? If so, the Louise Grace that turned up as a witness in 1916 may not have been a sister, but a sister in law,
If none of these are her, it may help if we knew the names of her parents, and any siblings (with their rough ages)
Roy G