Divorce---Dating HELP plz?

Parents are currently getting a divorce. Divorce is not finalized, noting that it is been 5 months since they ahve split up. My mother goes to divorce group meetings because she thinks she needs help coping with the divorce and **** going on with my father. She meets this guy at this group meeting, takes place 1 a month. Note that every single time she calls to amke an excuse saying shes comming home late and i have to watch my brother and sisters. She talks to this specific guy on the phone all the time, shell say shes going to be but then talk to him for 2 hours then go to sleep. She became obbsessed with emailing the guy and when she talks on the phone wiht him she flirts and acts like a giddy school girl. THAT AGGRAVATES THE LIVING HELL OUTA ME. I also caught her buying things online *personal-couple* things. Called her out and she kept denying it, which led up to a huge fight. Now tonight he admits to me, my brother and 2 sisters that she is going out with him. She keeps saying when ever i talk ab out this with her that she has felt like crap for the longest time with my dad and says he makes her happy, and makes her feel good about herself. I dont see why she needs to go off with another guy act like the way she does in front of me when i told her to stop when she has her 4 kids. Can any1 help me on what i should do. I am severly pissed off at her and that #%*!!*!#! that she going out with. My sisters are crying and brother and i are filled with rage. The divorce isnt even final yet and theres no cooldown time. Any advice on what i should do. Im 16 and full of rage and i dont want anyhting to go out of ahnd,