You sound like a poster who used to post her obsessively with these questions. Did you used to post here in the past?
There is no way for her to get pregnant this way, or any woman.
Here's the thing about obsessions. If a gynecologist told you he'd examined your sister, and found she's not pregnant, and in fact it would be impossible for a girl to get pregnant this way, you'd feel momentary relief for about a half hour. Then, the doubts would start to creep back in, you'd wonder if the gyno was right, could the exam have been wrong, etc. No amount of telling you this isn't possible will help in the long run - I would suggest therapy for obsessive thought patterns.
Is this new to you, to have this sort of bothersome thoughts, on any subject?
You need to tell your mother you're experiencing obsessive thoughts that are making you suicidal and keeping you from sleeping. You don't have to tell her it's about sperm - you can just tell her it's about germs, and it's taking over your life. It's the same fear, anyway.
Get your mother to take you to the doctor, and tell him/her what's bothering you, and get some help before you harm yourself.
Are obsessive thoughts new to you, Shounty? Have you recently had strep throat or other viral illness?