How to include stepparents when both parents walk bride down aisle?

I'd let them each choose someone to walk with!

My dad walked me down the aisle so my mom had my brother be her escort during the seating of the families. My father-in-law was my husband's best man, so one of our ushers (a close family friend of theirs) escorted my mother-in-law down the aisle!

1st thing: ask them.! They might want to be seated before the procession starts, if they don't like to get any attention.

They might want to walk alone or with anyone. The only don't : dn'tmake them feel uncomfortable by pressuring them to do something they dislike.