I am no trained psychologist, but I would say that your early years created this fixation that you have for your sister. In rubbing your penis against her backside, you may not have understood that this is a sexual act, but human instinct exists in all of us, whether we understand it or not. New problem sexual attraction can sometimes be very inconvenient. When one person is attracted to another, and that attraction is reciprocated, it can lead to a very satisfying relationship, even marriage. Unfortunately, sexual attraction does not always fall within the boundaries that society sets for us. Society considers that sexual relationships between close blood relatives should not be allowed. I can argue about whether society is hypocritical or not, and whether it has a right to impose morals upon us, when we should really be making our own choices about how morality, but whatever society might have to say, the law in India (assuming that you are from India) does not make incest illegal. In your case, you are under 18, and therefore considered a minor. Incest is tolerated as far as the law is concerned, between consenting adults, so if you did commit the act of incest with your sister, you would be breaking the law, and she would be more liable to prosecution then you, as she is considered the responsible adult, being over 18.
Where is this leave you? I'm not about to tell you what you should and shouldn't do, so whether you wish to pursue your sister from a sexual point of view is through your own choice. Incest was always prescribed because of the risks of pregnancy. It was always felt that a child produced from a close blood relationship had an increased risk of congenital and genetic defects. The statistics show that there is a slightly increased possibility from such things. However, almost all sex these days is for recreation, rather than procreation, therefore one has to question whether incest, when it is between people who are freely consenting, and is not intended to produce children, should even be banned at all. From your point of view so, how does your sister feel about you making sexual advances towards her? Is she aware that you take her used panties for your masturbatory fantasies? If she is, do you know how she feels about it? The fact that she is older than you, and probably, when you were first rubbing yourself against her, she understood what you are doing, because she was at bit older than you, meant that she had no objections to what you are doing, or she would have stopped you. Whether she would feel the same way now, and she is a fully matured young woman, you won't know unless you talk to her.
Let me put a hypothetical question to you: if you told her how you felt, and she said "okay, let's start having a sexual relationship." What would you do? And what would be the consequences further down the line? The biggest problem that we have in life is that we start things without thinking of the consequences. You are not in a position where you could finish up being married to your sister, so any relationship you had with her, if it becomes close and intensely sexual, would sooner or later have to come to an end. I suspect that it would come to an end pretty quickly, the moment that the two of you are caught by other family members. I can only imagine what fallout would be from such a thing occurring. So we are looking not only a question of can you do this, but for the safety of both of you, should you do it? And of course, we are only assuming that she would agree to it. The chances are, she would say no, and may in fact be quite horrified by what her little brother has in mind.
Incest exists, and a quick glance at the number of incest related questions we receive on this forum, shows that incest is not only A favourite fantasy, but is also a practice that is widespread. It is occurring in all levels of society, and everywhere in the world, regardless of nationality, race, or religion. I used to be surprised at the number of private messages that I was sent in regard to incest, but these days it doesn't surprise me at all. While society likes to pretend that these things are rare, the reality is that it is going on in a street near you, behind closed doors. It might be your neighbour, it might be one of your friends, it might even be others of your family members. I would think very carefully before getting involved in anything like this. Just because it is possible, it doesn't mean to say that you should do it. But in the final analysis, what you and your sister do will be down to the actions you both take.
I'm not sure whether this has been very helpful at all, but I hope it at least makes you think before you do something you might regret.