Here is the full list of all 29 Small Sister achievements.
Acquire End 5.
Win day 6 with Mark.
Be lovers with Nancy.
Win day 4 with Mark.
Win day 3 with Mark.
Win day 2 with Mark.
Win day 1 with Mark.
Win day 5 with Mark.
Win day 1 with Annette.
Win day 2 with Annette.
Win day 3 with Annette.
Win day 4 with Annette.
Win day 5 with Annette.
Win day 6 with Annette.
Be lovers with Mark.
Be lovers with Annette.
Be lovers with Barbara.
Be lovers with Paul.
Be lovers with Linda.
Be lovers with Brian.
Be lovers with William.
Acquire End 1.
Acquire End 2.
Acquire End 3.
Acquire End 4.
Acquire End 6.
Acquire End 7.
Acquire End 8.
Acquire End 9.