Hello and welcome. Thank you for providing us an opportunity to help you.
Have read your question in detail and let me try and throw some light on that. I shall remain concise and crisp so as to provide clear understanding to you and avoid any confusion.
To begin with, in any of the scenarios, i.e. you working full time, your sister in law not being employed at all or your mother being a pensioner, will have NO effect in how the income from forex trading would be treated.
The income will all scenario would be treated in the same manner. It would be chargeable to Income Tax.
Note that a person who regularly buys and sells large quantities of foreign currency with the sole purpose of realizing a gain. That person is trading. If what you do on the foreign exchange is a trade, then the profits will be taxable (and your losses will be deductible).
HMRC refers to 'trade' as running a business - not to be confused with trading/investing.So, let me answer your questions one by one.
1. Your income would be chargeable to Income tax at your applicable rates.
2. The profits / losses would be accounted for for tax purposes annually.
3.No tax implications on you if you manage your sister in law's or your mother's account. They would be liable for tax for their profits.
4. Your employment status will not affect how your profits are taxed. They would be added to your total income and taxed irrespective of employment status.
5. Yes, the expenses you listed would be deductible.
6. Yes, that would be the cost of capital -- loan interest would be deductible.
The below are some tax preparers listed. However, you should best search them locally.
I hope I am clear now and this would now help.
Let me know if you have more questions. In future, if you need to reach me directly, you may prefix your question with "For Rakhivasavada......" and you will reach me.