Multiple Partners

It had been about 1 years, I started a small business along with one of my best friend and her brother's wife. Everything was and is going fine till now except for few hidden relationships we share with each other.

My friend and her sis-in-law are both married and have been having a good married life. However in recent encounters we have been exploring each others. I have been constantly having sex with both of them one at a time, without letting other know about it.

Yes, I agree that I always wanted to have sex with my friend but her sis-in-law came as a surprise package. I have been trying to avoid being sexually active with them since they are already married but I just can't help myself when we are together. It almost feels as if I have been using them for my sexual pleasure.
Just confused what to do and what not to.