Hi Cassie,
You've got some good advice already but I wanted to add my two cents. Everyone at some point in their lives, no matter how wonderful the families are, has had to deal with some sort of issues.
When people don't know each other well or know each other’s personalities that well and are not family, things can easily be misunderstood or misinterpreted. I, myself have been misunderstood or misinterpreted before, so I know it happens to decent people.
I have an agreement between my sister-in-laws and mother-in-law that we talk about what ever it was that bothered us then move on. It has done wonders for the family. It wasn’t always like that though. I was once in your position, that is why I am giving you this advice.
I do believe in taking the high road. Do not let anyone make you less than your best. Do not let anyone make you angry enough to act, say or do nasty things. Stay in control, it shows better character. However, addressing issues doesn't mean you are not taking the "high road". Taking the "high road" is not acting a fool. You can still address issues and not be a fool. Be assertive, which is open, honest and direct. Do not name call, accuse, blame, etc.. just stick to the facts. And keep in mind there are ALWAYS two sides to every story, everyone has their own truth. Just as strong as you feel about your version, the other person feels just as strong about theirs.
With that being said, I think that you should try and resolve the issue between your mom and in-laws as well as the issue between you and your sister-in-law. Sweeping things under the rug doesn't always help the situation and only works for so long. Family gatherings are uncomfortable and fake, and after a while just not worth it (been there, done that). For those willing to resolve issues and move on, great. For those not willing to or not willing to change their behavior, that is their choice and they will just miss out on spending time with you and your wonderful children. Do not make special arrangements for those who are not willing to take a look at themselves and work at being a better person.
An example of how to address issues and stay on track:
"when you did this (explain issue), it made (person's name) feel this way (state feelings)."
Then I would suggest to the parties involved to be more aware and/or avoid that type of behavior in the future so there are no misunderstandings where someone ends up with hurt feelings or offended."
I have a simple decree that helps me be a better person all around. Be polite, be kind. The world would be a much easier place to live in if people just followed that rule. We often forget how following that simple act can prevent so many issues, fights, misunderstandings, etc. Just be KIND and POLITE! Don't take jabs at people. Don't try and look for something to pick on. Don't talk about people as soon as they leave the room. Try and only say positive things instead of finding every little negative thing about someone. You will find yourself feeling happier, better, like you yourself ARE a kind, polite person.
I say this because people are so eager to jump and attack each other. It's almost like they want to be angry or have something or someone to be mad at. Why are we always trying to upset ourselves? Are we all so stressed we are trying to use these little things as a way to vent?
I come from a VERY negative family. My family acts like everyone is out to get them. My family wants to raise hell when they think anyone disrespected them, etc. My family won't even take the time to find out all the facts or hear the other person’s side of the story; they just get all angry. It's very embarrassing and ridiculous.
Life is too short and too precious. I am sure you are very aware of that with the type of work your husband is in. You would think his family would want to get along considering how fortunate he was to come home safe. Oh, how often people take things for granted.
I thank your husband for his brave efforts and I thank all of the families, both his and yours for the sacrifices you make by supporting him in honoring his country and keeping this nation a safe place for me and my children to live. May God bless you and your whole family. I truly wish you peace and happiness.