When I started this forum last year, I knew that there were many wise and kind people with real life experience who would be able to offer useful advice to others who were confused, or suffering some kind of emotional pain, and that is exactly what has happened on this thread. BT read your question, understood perfectly what you are going through, and has given you some excellent advice which, in my view, you should hear, understand, and use.
Psycho-sexual development refers to the process of changing from an infant to an adult who is capable of relationship and sexual reproduction by mating with another human being. In many ways, that process is automatic, and does not need to be managed. For example, soon after a baby is born, it discovers its own genitals and begins to play with them, which is called "masturbation." This is an entirely normal and expected part of growing up (psycho-sexual development), which is needed so that the growing child can feel its own body and feel how it works. Learning to achieve orgasm by means of masturbation lays the groundwork for being able to have orgasms through sexual intercourse later in life, and so masturbation is required by the human being in order to become a fully capable adult. Unfortunately, religious authorities and others have demonized masturbation, labeling it a sin and the child who engages in it a sinner. This is completely erroneous. There is nothing at all sinful or wrong about masturbation. In fact, it is good for you!
The same is true of other kinds of sexual experimentation in which children engage with other children or sometimes animals. It is not a sin or wrong. On the contrary, this is a normal part of psycho-sexual development. Such experiences teach the child about her own body, and how sexuality feels. This prepares the child for later functioning as an adult. Now such activities, as I have written elsewhere, have one meaning when a child does them, and quite another when an adult does. For the child, sexual play with another child or even with an animal is experimentation. For an adult, it is not experimentation, but the sign of an unhealthy relationship to sexuality, for we expect adults to have had sufficient psycho-sexual development to be able to engage is sex with other consenting adults, not with children or animals. In other words, one sign of an adult is already having passed through the stage of childhood sexual experimentation--not to keep repeating it, which would be childish. I hope you see this difference.
I will repeat myself, Izzy. You have done nothing wrong. Everything you did with your sister was a normal part of childhood sexual experimentation aimed a furthering your psycho-sexual development. YOU HAVE DONE NOTHING WRONG.
I strongly advise you to simply accept this opinion, which is the opinion of a trained and experienced psychotherapist, and now just forget the whole thing. I advise you not to discuss it with your mother, which really will not help you, and which might cause her pain and confusion. It is up to you now. Everything you did with your sister was a normal and expected part of childhood development, not a sin or a crime. Please take my word for it and move on. When you are older and have had lovers of your own, I am sure you will understand.