Wow, this opened the door to my old memories. I know you are feeling pretty bad right now. In my case, I was 13 when my father decided to leave the house to live with another woman on December 8, 1994. What a date, huh? Just around Christmas, my birthday on the 24th, and leaving my mom with my 14 year old brother, 6 year old sister, 2 year brother and 2 days short of my baby bro reaching being a 6 month old and his first Christmas. Prior to that my parents where always fighting about him cheating, "working" late, etc., although he always denied it.
As you can read, it was true, and let me tell you it was HARD for all of us.
For my mom, I can't even describe her pain caused by the man she faithfully believed (she grew up strictly catholic) was marrying in the eyes of God for the rest of her life. The man she married and for whom she did not study medicine after getting an undergraduate degree, for a pact that involved her working to pay for the family to survive while he was the one who studied medicine. The one for whom she worked he *** off and helped build an extremely successful medical center from scratch. She started with him when he literally didn't have anything, and after 15 years of marriage, and after helping him reach success, he not only left her with 5 kids, but he also wanted her out of managing the medical center which she had developed with him. She felt and was betrayed on so many levels, either personally or professionally.
In my case, it was hard for me. I won't deny that some part of me was happy that he left because I thought their fighting will stop and thought that things will get better (childish wishful thinking). On the other hand, the situation led me to grow very fast. Suddenly I was having the responsibility of trying to keep my deeply depressed mom together, and trying to take care of my siblings. Let me tell you, my mom is an extremely strong person in general, but like a hard wooded tree in a hurricane she just fell down, instead of being resilient to the winds. Yes, she cooked food, and took us to school every day, but other than feeding and school, she just kept to herself inside her room. To this day, ~14 years later, she is ok but still feels rancor towards my dad, the woman, and his family, because while my mom cried every day to my grandmother, behind her back they knew about the other woman and even opened their doors to her for visits. They later accepted it when my mother found out and said they didn't want to, but what could they do.
So yes, I was totally devastated and cried my eyes out, but had to kept it together for my family. This has been a good and a bad thing. Good because I helped them grew into what they are today and my siblings kinda think of me as their second mom hybrid sister. Bad, because I grew too fast, and although I am doing my graduate studies and have led with my life (romantically and otherwise), I still feel a lot of responsibility towards them like a parent instead of a sibling (who else would go to every sports competition they participate with my mom to support them or drive them to their rock band practice, or drive them to the mall or movie theater when they want to go out with friends etc. lol). I guess a good outcome of that bad situation, is that now we are all very tight with each other, we value each other, and we take time at least one day a week to go out together, eat, shop, or just being together (mom + siblings). This make us stronger as a family.
I know this has been a long statement, and I am sorry. As I told you at the beginning, your situation brought back old memories. In a way I am explaining my situation to let you know that you are not alone in your pain, and at the same time it has been kind of a therapy for me :-). Yes, it still makes me sad if i think of it, but you can be sure that time will let you feel it less painfully.
My only advice is to be there for your mom and sister. Your sister needs you right now, especially starting an already difficult stage (teenage years), where anger and sadness, believes of unworthiness and instability, can lead to doing stuff that you may not wish her to do.
In the case of your mom, its hard, because if someone stays with the cheater is because: 1) she is ok with him doing it (wish I doubt if she screamed at him); 2) she wishes that it didn't happen and its trying to think that all is just a "bad dream" (wish is possible, because no one, especially someone married for 28 years, wants to think her life is not what she thought or planned to be); OR 3) she is just acting like that because she thinks that both of you still don't know and don't want to hurt you and its possible that she may or may not be planning to leave him until getting more legal or counseling/therapy advice. So talk to her, let her know that both of you are aware of the situation and ask her how does she feel and what she is thinking.
About self-respect, yes is important but for some