Walter Lovick Butler did have a sister, Alice Louisa Butler.
IGI extracted baptism for her:
ALICE LOUISA BUTLER
Birth: 06 DEC 1866
Christening: 05 JUN 1867 St Sepulchre, London, London, England
Father: FRANCIS RICHARD BUTLER
Mother: SARAH
She's with her parents, aged 3, in 1871 (Alice Butler): RG10/605/47/34
Still with parents, aged 14, in 1881 (Alice L. Butler): RG11/330/32/57
Not with parents after that. Dad, Francis R. Butler is in the City with his second wife Mary Ann in 1891:RG12/239/5/11. He died Dec qtr 1893 aged 66.
So what happened to Alice Louisa?
She is not the Somerset-born Alice Louisa Butler who married Robert Derisley, Dec 1890 Kensington.
She is not the Kent-born Alice Louise Butler who married Robert John Castle, Mar 1890 Thanet
I think she needs to be eliminated one way or another - otherwise I might start developing
entirely speculative and wishful theories that she was the Alice Butler who married Horatio
Talbot Jun 1919 Camberwell, and that this sealed a long-standing relationship in which she'd given birth to Horatio's illegitimate son Fred in 1898 and had to leave him with her brother and his wife
Edited: OK I think we can forget the speculative theories - Alice appears to have married Henry Ridsdale Jun 1886 Bethnal Green - family in Southwark in 1891: RG12/340/72/68. Looks like Fred's mother will have to be found elsewhere... 
Anna