If a sponsor funds your trip to the UK, there are four simple rules to follow:
A person not related to you cannot be your sponsor. Your expenses can be covered by parents, siblings and children, i.e. your close family members.
A sponsorship letter alone is not sufficient evidence. Your relative must show that they have enough money to pay for your trip by providing a letter from their employer and/or bank statements.
Your sponsor must show that they have more money than you would have had to show should you pay for your trip yourself. The logic behind it is that your relative must also have additional funds to support themselves.
Irrespective of your sponsor and their money, supported by necessary documents, you must also demonstrate that you have funds at your own disposal.