My mom walked in on me...?

Don't "confront" your mother. Tell her you'd like to talk to her about what she saw. I'd be candid and tell her that you were masturbating. This won't come as a shock to her since most women know that all boys do that. They may not participate themselves, but they believe that all young men have a need to spill sperm. Tell her you'd appreciate her not telling anyone, especially your step dad, since men are sensitive about other men knowing they need relief by hand. It is kind of an admission that you can't get a female to relieve your needs. Tell her you weren't involved in cybersex, and just needed relief at that time. She should understand since women think males think of sex all the time. If you let her know this honestly it may open you both up to a lot more honesty between you and even some mutual understanding of what both of you need.