Hello
Visitor visas are not an exact science. However, there are some usual refusal arguments often brought forward by the Home Office. You want to be proactive and avoid getting within the scope of those refusal reasons.
You can always apply with her husband's bank details. But I have seen refusals formulated around the lines "While I am satisfied that your husband earns such and such money, it's ultimately your own personal circumstances that I am considering in the context of this application...". If the Case Worker pulls that card, your application is dead in the water.
I know that in Pakistan many women, especially elderly, don't need or have a bank account. However, as you are in the UK and there will be occasions for her to visit you, it might worth considering to open a bank account for her there. So rather than using cash, she can use a bank account so she will have a demonstrable income.
But the situation is what it is. You can't make it different. Just add a support letter explaining the purpose of her trip and be honest. There is no need to hide that your wife is pregnant. Helping family is not working illegally. Otherwise if a visitor prepares a breakfast for you, the Home Office can arrest you all
Just send the application and hope for the best. It seems on the weak side to me but it still something worth trying.