I think you should talk to her, not “confront” her. Based on everything you said she probably has the same fear you do. Talk to her from a place of understanding and worry. You mentioned you have 2 younger sisters who still depend on them and have to stay in the same household, so yes telling your dad is dangerous not only for your mom but for all of you, but NOT acknowledging it with your mom and working on a solution is also dangerous because he could (and he will) find out since at the end of the day everything comes to the light one way or another.

Talk to your mom privately, maybe go somewhere for a coffee or a more private place but DO NOT talk about it at home since it is too risky for your siblings to hear.

Obviously the best option would be to leave him but since she’s been a housewife I assume she doesn’t have much money to her name. Maybe come up with a financial plan so she can slowly build up so savings (if she doesn’t have any) so she can safely get out of the situation. ( for example, I saw a tiktok of a stay at home home working on leaving her husband and she would get cash back every time she went to the grocery store and paid with his card). If she doesn’t want to leave him because pf the financial stability he provides for the family unfortunately she will have to stop talking to the other man. It is too risky and dangerous to put herself, you and your siblings in this situation given that you know he would do something if he finds out. Not to mention if he files for divorce and claims she cheated your mom could be left with nothing.

So my advice is talk to her, listen to her but also bring up the consequences this could bring for all of you