It can be very difficult for anybody if they belong to a family where people are more "successful" than most other people. I feel sorry for people like Abhishek Bachchan, Rahul Gandhi, MK Gandhi's sons, Rohan Gavaskar, etc. because everyone expects them to be as "good" as their parents. Everyone is different. Everyone is different. You are not your "over-achiever" sibling. You are a different person, so please stop comparing with your siblings.
I had an acquaintance in Minnesota who was going through a similar situation. He was in his 3rd year of MS and had not told his parents that his GPA was 2.75 and he required 3.30 to pass. I tried to give him courage to tell his parents, but he could not muster up the courage. So, the longer you wait the harder it will be.
So, my recommendation is that you honestly tell your parents about your current situation. I know it is hard, but it is much easier than doing it next semester and much much easier than doing it the next year. They will eventually come to know, so it is easier to be honest right now rather than carry this guilt for a few years. This guilt might even hamper your studies in the next semester. So, tell them today. Tell them now.
Tell them how you are lacking the motivation to study and seek their help. They love you more than you think, so they can help you more than a stranger like me.