Why has my sisters personality completely changed

How can a person change personality? My husband left me 6 months ago, but he had changed for about a year before he left. I worried sick about him, thinking he was having some sort of breakdown as our finances are a nightmare as we bought a house we can no longer afford to live in. He is self employed, and started going out at night and switching his phone off. I was beside myself, but kept it from my kids as much as I could. I never suspected for one minute that might be with other women. He is so plausible and assured me that was the last thing he would ever do. I looked after him as best I could, and he let me. I only found out 2 weeks ago, after us talking for the last 6 months about him coming home, that he has another woman with 2 kids similar ages with ours. I am disgusted - I have been lied to by so long and I believed everything he said to me up until then. The husband I knew, a kind caring man, is long gone. Now I'm terrified he will stop providing for our home and kids. Life used to be good.