meepop said:
My mother unfortunately has MS, Multiple Sclerosis; it's not always easy for her to walk.
EDIT3: To clarify some things: We don't know if any sexual happenings are occurring. We only know that my dad is going over her house, and spending the night there. Also, my mom hasn't had MS her whole life. Only since 1998 when my sister was born.
I think that's likely the issue right there.
You say "only" since 1998. That's 14 years, since you were two years old. You also said she has trouble walking. That means the disease may be getting fairly advanced.
You mother may no longer be able to have (or enjoy having) sex.
If so, she and your father may have agreed to let him get his sex elsewhere. That isn't an uncommon arrangement when one member of a couple gets a chronic disease that can affect that individual's ability to have sex.
You're 16. I know this all seems upsetting to you, but how do you think it feels for your father? The woman he loves is having her neurons stripped bare. She's losing herself, which means he's losing her. Slowly and painfully.
You're only 16, so they might be keeping things from you that they think will upset you or that you're not old enough to understand. They might not realize how much you're already aware of.
Best option - try talking to your parents. Not accusing, yelling, or fighting - talking. Don't accuse anyone of anything, just talk about the situation.