In all cases, you need to show that you are completely or mainly financially dependent on the UK sponsor.
Other deciding factors will include demonstrating that you have enough money and suitable accommodation where you can all live without help from public funds.
In some cases you will also be required to show that there are no other close relatives overseas who are able or willing to support you and that you have sole responsibility to look after the person for the duration of their stay in the UK.
Registered with the office of immigration Services Commisioner