My girlfriend may be sleeping with her step-brother, how should i handle this ?

Ive dated this girl for several months after the first few dates she was very worried about bringing me inside her home where her step-brother stayed with her, he slept on the couch in the living room which i found kinda weird cause the house has 3br and basement,as time progressed i visited more often when we had our sexual encounters at night she would want to suck my fingers during the whole time to me i felt that she like threesomes with two guys not girls, he would listen on the couch,time moves along he sends her a txt one night saying there a party? she responded where? he said in you mouth im cuming! lol, i didnt find this funny at all and not a joke i would send to my sis i have 6 so i should knoe,we are 30 her stepbrother is like 38,so everybody grown enough to know this raise serious flags,one day i go over after i get off of work i come through the living room to the kitchen to see him darting outside the back door with her standing there looking rather suspect, like he was getting head or something, her other step-brother who is in out of jail ask him what happened like he knew what was going on like i interrupted the family gathering,I ignore it because i don't want to accuse them of something like that time goes on and they get into a fight like full blown fight i wasn''t there but she kicked him out and moved to different house he moves in with there mother,they have since squashed the beef and are cooler than ever he has been giving her money now not allot but some, she spends most of her time at her moms where hes at shes over there at all type of times when her mom is at work and she think I don't know her mom works , shes now lying saying she go to work on certain days she don't or will leave extra early with like lotion and extra clothes to her mother house while she is at work of course,I have pretty much broken up with her because of the lies i caught her in, but she claims she a saint and nothing happened I also confronted him about this and told him how i felt and that i felt that they were sleeping around and he became very angry but did not fight me nor did he tell his sis we even had the conversation this is all very odd so i need advice rather good or bad intelligent or ignorant. thanks