Here is a massive thread about pillow humping.
http://ehealthforum.com/health/topic116438
.html
It is just a form of masturbation. No better, and no worse than any other way of masturbation.
Masturbation is nothing to feel guilty about. It is something private, but not something dirty or evil. We were created as sexual beings. If it was not for that, the human race would have dies out long ago. You have been made with a wonderful, sexy body, meant to attract men and sex is wired right into your brain and behaviors.
Masturbation is a way for you to learn about your body and sexuality, it is extremely healthy for you, both mentally and physically, and it is a healthy way to manage sexual desires. If you feel guilty about it, or think it is dirty, it is however no longer healthy for you. It will affect your self confidence and self esteem. And that is bad.
You already know that your sister masturbates. The chances are very very good that your mother does, your aunts does, your teachers does, your friends does, the women you see at the mall does, women that read the news on tv does, women that appears in movies and tv programs does ... It is nothing extraordinary or exceptional. It is just a part of normal life for billions and billions of women. How you masturbate is your own thing. It is nobody elses business. As long as you are safe, it is fine.
When masturbation does become an issue is if you concentrate on that so much that you do not participate in your normal activities or interact with other people because you would rather stay home and masturbate. The other reason why it can be bad is if you feel guilty about it and beat yourself up over it.
So relax and accept it as a normal part of life. Keep it private but do not feel guilty or dirty about it. Read the thread I linked to above and see how many other women does it. Then at least you will know that you are not alone.
Take care!