I am lucky enough to have been married to DH for many years.
I was worried about introducing my MIL to my mother as my mother was a socialist & stood as prospective MP for the Labour Party in an unwinnable SW seat in the 1980's. She was chair of the SW Labour Party & a big cheese in her day.
My MIL was a dyed in the wool Tory & had opinions that I knew my mother would not agree with.
However, on meeting, they discovered that they had a lot in common - they both loved Gilbert & Sullivan (G&S) operettas, they both played Bridge, Scrabble, crosswords, Countdown etc.
Once DH & me started having children, they shared the love that they had for GC & celebrated the successes of their shared GC.
They became the best of friends as they shared principals - they were just coming from them from a different direction. For example: they both agreed that people who can should work & those who genuinely can't should be supported.
As life went on & they were widowed became unable to drive, we would get a summons from one, or other of them, suggesting that DH or me accompany them & drive them to a G&S matinee. We would discover that they liked to have a G&T before the performance, a large glass of wine in the interval & a 'snifter' ato close the afternoon. I was happy to do this as I'm also a fan of G&S, but dealing with a couple of tiddly elderly ladies was sometimes a challenge(!)
Both of our mothers are long dead. However, DH & me have been remembering how we would be called upon to transport one of our mothers to the another's house during Wimbledon as they enjoyed watching it together whilst they sank a load of G&Ts saying: 'Just a small one for me dear' x5 (yeah as if...) + always a brandy in coffee or a milky drink for the road.
Again dealing with tiddly old ladies. But they'd had a good day, I can't & would never complain.
They became the best of friends because they were able to put political differences aside & focus on what they had in common.