My husband of 39 years recently passed away. At the wake I asked that my children sit in order of birth with their spouse, then my mother-in-law and sister-in-law (the only living immediate family). When people came to give condolences they would be met by my SIL first, MIL, youngest son, oldest son and spouse, daughter and spouse and then I would be last (closest to the casket). My mother-in-law has not spoken to me since. Did I do this wrong?

If they did not like it, that is their issue. But it may just be the death, and they not able to deal with it.
Why not call and ask them? Most areas of conflict can be handled by communication.

To be honest and blunt, if anyone gets upset about a seating arrangement at an event like this, they are petty and looking for a problem

Agreed. The seating arrangement can be whatever you choose it to be or just let people sit where they may. Most people know the first several rows are meant for family anyway. Plus you have more on your mind than whether people are happy with their seat. Anyone not happy is trying to make the event about them and that's pretty petty to say the least.

It would have been best to have the funeral director arrange the seatings, in person. That's what you hire them for.

It would have been best to have the funeral director arrange the seatings, in person. That's what you hire them for.

Totally agree. Why should bereavement be complicated.

Your mother-in-law has issues, and just looking for a conflict, don't give her one. I'm 53, been to a lot of funerals... including my own fathers... and I have never heard about anything but immediate family is up front. Never about a pecking order by age.