There are many different poems that can be engraved on a headstone about losing a sister including "You are always in my thought dear / Sister I will always hold you near" and "Our happiness together / Will be our cherished memories forever." A headstone will usually not denote that the deceased was a "sister" because she was also a "daughter" and may have been a "mother" or "friend" to others.
For this reason, most people choose sayings that focus on the love and affection held for the deceased. Another poem to be inscribed could be, "Sister as we lay you down to rest / We pray for nothing but the best / Your love we will always hold dear / And we know that your spirit is near."
Poems can be personal or can be existing poems found in poetry books. Personal touches can be added to the inscription after the poem as well. Another poem that is fitting for a sister would be, "God's greatest gift returned to God, my sister / O Lord I put her in thy hands / She had a kindly word for each / and she died beloved by all / She lives with us in memory / and will for evermore."