Families are a hard thing to deal with. You are a graduate with a BA degree in Sociology. I would hope, that with your BA degree in that field, that you would have the working skills to work this one out. If not, please let me give you some advice that you may not want.
Tossing a little comment that your sister is not saved, was a cop out on your part. You profess to be a Christian, so turn the other cheek. I'm serious. This of this: So what if you feel that your sister and sis-in-law are friends. I'm not friends with either one of my sis-in-laws because we do very different things. They are apparently friends and do things together that I am not involved with because they are always trying to out do each other... their problem, not mine. I personally have much better things to do with my life that be competitive with another woman.
I am a house painter and they are office types. I wear blue jeans and they wear skirts. We have nothing in common. At family gatherings, I talk to my aunt, my parents, my brothers, my own adult kids and I will talk with my nieces (who are adults but can barely converse in public) and nephew, and if one of my sister-in-laws engages in conversation, I converse. If they have issues with that, then tough. End of sentence.
Today will be our "Christmas". If one of my sister-in-laws thinks that she needs to tell me about my husband's and my adopted RAD (Reactive Attachment Disorder) daughter, to tell me what that girl (adult) is doing these days... (this girl put us through total misery, and threw away her own baby boy, but now she lives with some actor in LA... the entire story is very long), I intend to firmly and quietly tell my sister-in-law off. I know that the woman will be quite vocally offended, gasping, spitting and sputtering and all that, if this happens. Her problem, certainly not mine.
So, accept your family for who they are and don't get twisted up with them. You can find a more socially acceptable group of women friends to hang out with - go camping with. I did, and I'm much happier in the very long run.
I am 54 years old, married to the same man since the both of us were 18 years old. We are high school sweethearts and married over 35 years. (Yes, it takes a lot of work to stay married for that long - both parties must stay committed). We have two sons, ages 33 and 30. I am a grandmother of a nearly 5 year old girl, and a 3 month old boy. I am a house painter of 35 years. My husband is a building contractor. I have been a volunteer firefighter/EMT/Ambulance tech (10 years). I rescue equine (20+) years. I was also a beautician many years ago too. I also volunteered for the Bureau of Land Management's Wild Horse and Burro Program and was their "volunteer of the year" in 2006.