Apply. Thats what i will say. If you are eligible or not is the VO decision.
But if you want suggestion, forget for now how long your actual course is. Make an SOP and tell them about it. First begin by saying why you decided the 3 months course. And then continue from their and explain how you will continue from the end of 3 months to the 3 Year part.
However you may just get the student visa for 6 months. After you finish your 3 months course and continue it to the 3 year section, you will need to apply for a study permit extension.
The choice is yours. You could be eligible for the 3 months course but you will only get permit for 3 months and will have to extend it later. That or you could re apply for the 3 year course like your sister did and get an acceptance letter with 3 years date written on it.
