Documents for re-applying after U.K. visa refusal

I invited my parents, sister and nephew to come over for a post wedding celebration this July.

My parents got their visa but my sister and nephew were refused.

My sister was refused on grounds of no sufficient funds, and employment status is inconclusive. My nephew is under 18 and as his mother was refused ECO refused his visa too.

Questions are:
My sister’s salary goes into a different bank account and she only puts in savings on a different account and total savings would be around £540 at time of application.

However she is a member of their company’s Savings and Loan Assoc and has a savings of £3200 from the last 10 years she’s been working with the company but would rather have that money there instead of transferring into her savings account. Will the ECO accept that as proof of sufficient funds?

I am sponsoring the whole trip and has submitted relevant documents and ECO is satisfied with my capacity but would still want proof of my sister’s financial capacity which I understand.

What I don’t understand is they found my sister’s employment document inconclusive and ‘cannot be satisfied that she is employed as claimed’. My sister works as a Store Manager in retail and has been with the same company for the last 10 years.

Only thing I can remember is that the week before the application, she has been promoted to become Senior Store Manager to run the busiest branch of their company in the Philippines(44 branches in total) and next step for promotion would be to become an Area Manager.

If Certificate of Employment stating her start date, salary and position and request for time off for 3 weeks to go on holiday to visit family in the UK is not enough what other documents can she provide?

She included latest Tax Return in her previous application but was returned to her and was told it wasn’t needed.

Lastly, if they are questioning genuine reason for this visit, would it actually help if I send copy of hotel bookings for the wedding celebration? Reason for the celebration is that we got married in the Philippines last year and since none of my husband’s family was able to go, I would like to have a celebration including my family this time.

My parents are really elderly and would really help to have my sister around on their way over to the U.K. when sis and nephew go home after 3 weeks, parents are staying for 3 months and I can go home with them to accompany them home on the long flight home.