Hello, Escapists. So for the past few months, my dad has been going out on Sundays and staying overnight at some girl's house. The distance isn't exactly short, either. I'm 16, and my parents are still together. He outright told my mother a while back that he was seeing her and assumes that's fine; he spends most Sunday mornings and afternoons with this girl, until Monday or sometimes even Tuesday! He even sometimes does it again later in the week!
And that's not the tip of the iceberg. My mother unfortunately has MS, Multiple Sclerosis; it's not always easy for her to walk. The grass was getting very high, and my dad used to cut the grass on weekends. He now rushes me and my sister out on Saturdays to "spend time with us", preventing him from feeling guilt when he leaves on Sunday. Anyway, because the grass was so high and I was sick, my mom went out with a fever and MS and cut the grass. This is pathetic.
We never decorated for Halloween or Thanksgiving because he's the one that normally gets the stuff down, he lies about going up his mother's who lives up the street from us to instead go to his girlfriend's. He even said he'd help my sister with homework (She's two years younger than me) more than once, and ended up just skipping some math problems she had because he wasn't there. He's also lied to my mother more than once about this.
TL;DR and otherwise; My dad's basically cheating on his wife, and it's affecting all of us. Is he justified at all in doing this? My mom doesn't yell at him or really do much to stress him out as far as I know.
EDIT: Can anyone also tell me what I should do? I don't want to approach him on the subject because I don't know what to say.
EDIT2: Both my sister and myself are in expensive Catholic high schools; my father is the one who pays for them. And my mom is, again, unemployed so she doesn't have much money to live off of.
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.
EDIT4: My dad has been divorced once before; this was my mom's first marriage. Both of them are religious, but I somehow doubt that he would be against divorce if he's done it before. The reason I posted here was to see if there were any other ways of thinking about it, that he may be using to justify his reasons.