My sister got rejected because she applied as a dependent to my brother in law. He got rejected as he claimed to be a salaried director of a company, but failed to give a payslip. as a result the reviewer tried to associate his claimed income with the income of the company which did not add up.

further more, he claimed that he has parents living in the same country as him, but his rejection letter also states that "documents donot demonstrate what family members you have in the country". This is confusing as I am unsure what documents they expect in order to show family ties

 

I'll be honest - The team deciding visit visas are highly biased, and because these decisions are subjective with a lot of leeway to refuse based on personal intuition, they tend to make a very instinctive decision almost immediately, and then find a way to justify their decision. This was corroborated by a former UKVI caseworker (verified) on Reddit. So if the caseworker is intent on refusing you, they will usually just find a way to do it. Bangladesh is considered an extremely high risk country for immigration overstays/illegal working, unfortunately that does count against her (if unfairly) and whilst they do say "all applications are judged on their own merit", the truth is that previous refusals do cause greater scrutiny in subsequent applications.

Besides that, there is another big issue here, and that's the fact that they've stated your mother hasn't provided enough evidence. That would not be seen as a "mistake" by the caseworker (therefore there is no real reason to submit a formal complaint), it is something she needs to address by providing evidence which better supports her personal situation. My notes:

1. 29 days a long visit and requires significant funds. But, they do seem satisfied by the sponsor's documents, so it doesn't seem that her ability to fund her trip is seen as problematic, thus it seems they think she is not particularly at risk for seeking illegal work in the UK. The issues are more with proving significant ties to Bangladesh (primary), and evidencing her own financial situation (secondary).
2. You state: "additionally, my sister and brother-in-law were all based in the same country. so they are valid ties for her to go back " - This isn't how the caseworker sees it. If they're all travelling to the UK, then they cannot count one another as ties to their home country. They will have to provide other evidence of plans to return to the UK; e.g.: ongoing employment or studies, family members (e.g.: children or elderly people) who require them for care. This is especially important because her immediate family are travelling with her to see other close family in the UK, so from the perspective of a caseworker, why would any of them return to their home country. Compounded by point 4 below.
3. Your mother needs to provide evidence of the other sources of funds which are being deposited to her bank account, because her balance/incomings don't align with her retirement allowance.
4. A utility bill is clearly not sufficient to describe where her husband is currently present, so yes I do think that she should supply recent evidence that he's employed. Get a letter from his employer stating that he is currently employed and hasn't planned any leave for that time, if needed. He should also supply a brief statement that he is not travelling with her and will remain in Bangladesh. I don't think an expired passport will be particularly helpful because that doesn't mean he doesn't have another passport, or couldn't get one.

Yes it's a lot of evidence, but the unfortunate fact is that a retired person from a high-risk country with very strong family ties to the UK is already kind of rolling a boulder uphill.