What Is The Solution?

The OP description reminded me of decades ago problem of how our kid felt excluded by a group of other kids. I can not remember how we got through that. That was also the time with no google to look up things.

And these days, when we need something technical deciphered, we ask the kids. Perhaps they have solutions to deal with the "mean girls" problem.

The followig may be useful to help the inner child also.
Six Ways to Help Your Child Deal with Social Exclusion

 

Why are you having so many gatherings during Covid time?

 

Never! You should never fight for someone’s time, love & attention. If it’s not freely given, it’s not worth having. We cannot force relationships.

Give a break to yourself and these parties. If they both are getting along very well, let them. You have no control on that or any relationship wherein you are not a party.

I know it hurts to be left out in your own family gatherings. It hurts real bad. But do not expect anybody to realise the pain you are going through, when you don’t tell them. It's not fair to expect your BIL to “correct his wife’s mistake”, esp. when he doesn’t have a clue of what’s brewing between the ladies of the house.

If you are comfortable enough with your BIL and co-sis to let them know how you feel, then do so. Preferably, when you receive the next party invite. If you don’t have that comfort, then you have no much choice. Party or peace, you have to pick one and move on. Forcing yourself to be accepted, going out of your way to get their attention, finding topics to strike a conversation and include yourself ... you cannot do all these for long. There is something called dignity. Do not stoop below that. Not worth it!

 

Learn to enjoy your own company, they will come to you!

 

I have observed this at parties. I recall times when I was sitting alone but obviously enjoying my own company and at peace, looking around confidently, not paying undue attention to the samosa and appetizers in my Corelle or Dixie quarter plate. : ) Sooner or later, another guest would come join me or at least stop by and chat on the way to the food area.

Long ago, at weddings, I used to often find one distant aunt or someone sitting by themselves but calm and content. Such people make others curious about their calm demeanor, I guess.

 

I lived up to my late twenty's in India, and was living the 'IT" life, so obviously there was no concept of a "party" . Before marriage, the few times I have been to pubs, it was always with friends .
But after marriage, when I went to weddings (especially in-law weddings), the husband would leave me hanging and join his relatives and I would often be left sitting around not knowing what to do (these were pre-smart phone times, so no looking at phone either). These times, I have seen these "distant" aunts, just "people watching" with a smile. Later, I started doing it too, just watching people going about and trying to guess their backstories. I could not believe how entertaining it actually was!!!