Sorry to know your sister-in-law was refused for the 6th time. Unfortunately, it was expected.
She is young + no travel history + this time, you provided evidence that her other family members have TRVs, etc.
even though we provided job letter, working there since 2011, not sure what more ppl have added to prove ????
Based on your posts, she been working since she was 16 years. She could have demonstrated stronger employment ties than just the job letter.
A bit of advice:
Do not reapply for the 7th time. Also, it won't help to order another set of case notes.
She must establish stronger ties to her home country (employment or maybe enroll for a study program) + maybe travel to countries closer to where she lives, etc