Best advice. Players start the game knowing how and why they are going to stick together.
I'm also inclined to put my thumb on the scale a little as DM and give the players a loose connection that they can build on and incorporate into their characters while building. BG3 did it really well - everyone has a tadpole in their head, y'all gonna be mindflayers if you leave the group.
I recently had players all start as fresh recruits in an organisation - they got to decide the organisation - where the higher-ups put them together. Previously I did a one shot at level 5 where players already had an adventuring group together 20 years before and were called back together for one last mission.
Disagree with some of the commenters suggesting this is something husband should have taken to the grave. If OOP is going to be interacting with one of husband's exes, she needs to know it. OK, that conversion would go a bit weirder than most with the whole stepsister angle but nevertheless she has to know.