> So my sister was saying that AIDS is just symptom so it should be caused by more than just HIV.

I think your sister is unclear of these terms and how they are used. HIV is the virus and therefore, the cause. Its effect is a group of symptoms, collectively called AIDS.

Currently we have another virus that is all the rage: It is called SARS-CoV19, and its effect is Covid - though these terms are used somewhat interchangeably.

There is no such thing as "just" symptoms.

Also, only HIV causes AIDS. This is because AIDS means "Acquired Immune Deficiency Syndrome", with the "acquired" pointing to the fact that you get it from the "Human Immunodeficiency Virus".

There are some other types of immune deficiencies which can cause sort-of similar symptoms but they aren't called AIDS because they are not acquired from a virus.