Sister's best friend

I'm doing okay career wise and spotty at best relationship wise, I'm 2 years older than my sister and her best friend had just broken up with husband and they would get divorced. She was left with a small child and moved in with my sister, till she got on her feet. The husband departed and stopped paying maintenance and that left her struggling financially, she couldn't afford daycare to work. Then my sister also lost her job and had to give up her rented apartment and she moved into my 3-bedroom house along with her friend and child. Not a worry to me, anyway my sister soon got another job and as I worked from home. I was alone with her friend a great deal, one thing led to another, we started sleeping together. We had been fucking almost twelve months before my sister realized I was fucking her friend. That was the time I asked her friend to marry me in front of my sister. She accepted and sister was totally shocked, move along another 7 months we're married and she pregnant. My sister being her maid of honor, my sister had been thinking of getting her own place before I asked her friend to marry me. She seems quite content to stay with us, especially as I added another bedroom with Ensuite and a Ensuite on the master bedroom as well.