I need some advice!
My sister is the Maid of Honor, my brother's wife is a bridesmaid, followed by two college friends and two childhood friends. I like my fiance's sister, but I am not as close to her as the aforementioned ladies, and I have known her the least amount of time. My fiance did not choose my brother to be a groomsman for similar reasons, so we didn't think it would be a big deal. He has graciously agreed to be an usher. We have selected Fiance's Sister to do a reading for the ceremony, which she is fine with and said she would gladly do whatever we needed.
However, my mother-in-law was not pleased. She called my fiance in tears, wanting to know why her daughter was not a bridesmaid. She said she it was an unwritten rule that all siblings were supposed to be included in the wedding party. This was news to both me and my fiance.
Of course now I feel awful! I don't know what to do, but I have a few options:
1. ASK FIANCE'S SISTER TO BE A BRIDESMAID
I haven't asked all of my friends to be in the wedding yet, so no one would know if I replaced them with Fiance's Sister. However, I feel that extending an invitation now would be seen as disingenuous, and she would know she is taking a spot previously dedicated to one of my friends. Also, if I do this, my brother will then be the only sibling not in the bridal party.
2. SEVEN UP
I really don't want to do this, as I think 6 is pushing it, but we could include Fiance's Sister as well as my brother if we had seven bridesmaids and groomsmen. (Fiance has already asked his 6 groomsmen, so we can't do any replacing on his side.) I dislike uneven wedding parties.
3. LEAVE EVERYTHING AS IS
I feel the damage has already been done and there is no graceful way to fix it. I was planning on inviting Fiance's Sister to the "getting ready" portion of the day, so she would be included in a more exclusive group. Fiance is going to do the same with my brother. (Unless that is some sort of tacky faux pas!)
PLEASE HELP ME! I've been turning this over in my head for days. 
Re: Bridesmaid Crisis & Unhappy Mother-In-Law
#3. It's really more of an honor to be a reader than a bridesmaid as she will be standing alone and serving both the B&G. Would your FI like his sister to stand on his side?
Your FMIL should get over it, especially if FSIL is fine with being a reader at the ceremony.
His sister is ok with it, so his mom should be too.
ETA: My brother was not a GM when we got married. I'm still close with my brother. Tell your FMIL that the internet stranger said it's okay.
I just wanted to add that I also think Option 3 is the best. She is a reader, and that is exactly what we tell people to do when they want to include someone who isn't already in the wedding party.
FMIL needs to chill. Tell her everyone is happy the way it is and move on. Happy Planning!
Besides, think about it this way. Let's say you're FSIL and you have been asked to do a reading at your brother's wedding. Then, out of the blue, the bride asks you to be a bridesmaid later on. Chances are, the FSIL will find out that the mom bullied the bride into this. How shitty would you feel to know that you're only a bridesmaid because your mommy made the bride do it?
"Love is the one thing we're capable of perceiving that transcends time and space."
I'm the fuck out.
FMIL is being dramatic. This probably isn't the first time to pull a stunt like this, and it won't be the last.
FSIL probably doesn't even care and probably doesn't want to be a pity add bridesmaid. A reader is just as much of an honor.