I’m so, so sorry that you find yourself in such a difficult position. The first thing you do is stop doing a whole lot of what you’re doing. Your mother’s text messages and chats are her business, not yours. You already know enough. Reading more of them only makes you feel worse.
I don’t know if your mother asked you to keep this secret or if you’ve taken that burden on yourself. Either way, it’s much too heavy a load for a 15-year-old — even for a smart, sensitive teen like yourself. A heart-to-heart talk with your mom is way overdue. Tell your mother that you know what she is up to and that it is hurting you terribly. Let her know that you feel like you are caught in the middle between two parents you love and that it is just too much for you to handle anymore. Ask her to be honest with your dad and to either work things out or not but not to keep you and your brother in such an emotionally difficult situation.
If your mother asks you to keep the secret, it’s really unfair. Tell her that it hurts you to be asked to betray your dad the way she is. Make no promises. If this gets to be just too, too hard, think about whether there is a relative or another trusted adult you can bring into your confidence who can give you support and advice.
If your parents do divorce, it’s very, very important that you understand that it’s not your fault for getting this out in the open. Clearly your folks have had problems for years. Those problems have nothing to do with you and your brother. You can still have two parents who love you even if they don’t love each other.
I wish you well.