how he got married first

This reminds of that Fresh Prince of Bel air where that happened

It will make holidays so much easier, only 1 set of parents to celebrate with.

Kinky

Not only step sister, but is the mother in law still married. If so then it's gonna be an even bigger mess.

Location

But one less separate family home to have to visit for holidays. Nicely consolidated instead!

And your children will be their own first (step)cousins. My dad was when he was a boy and his paternal grandfather married his maternal grandmother after their respective spouses died.

No shit.

Sounds like the beginning of that song I'm my own grandpa

My actual wedding is April 1st....

When it pains, it roars.

My father married my oldest brother's wife's mother. All four of them live in the same house. Well one property, with a house and a "guest house" where my brother lives. APPARENTLY they all get along fine but it creeps me out to no end.

Simply put his wife will become his step sister

Well they're not really brother and sister, so I don't see the problem. Marriage never meant much to me though.

Not quite #59. Step siblings are children from previous separate marriages where one of their parents gets married to one of the parents of the other. So once OP's father marries his wife's mother, he and his wife become step siblings.

and his father also becomes his father in law

It only results in a massive family fight when the parents try to force the kids to break up under the conditions that "it's wrong for siblings to be in a relationship". And yes that does happen.