Ok, well, it's a worry.
When my dc were tiny, the options would have been: my db (we aren't close); my dbil & dsil (no kids of their own); my bff & her dh (reciprocal agreement, both families had young kids).
In the last decade ALL of those people have experienced huge changes in their family situations. Divorces, new marriages, new babies etc etc. Sadly, I would no longer trust my oldest friend with a house plant, given her choice of second husband.
The thing you really need to be aware of is that your wishes are not the end of the process here.
Assuming db AND dbil/dsil both express willingness to take on your dc, social services will decide who is best able to look after them. Your opinion will not be all that important, because, well, you'll be dead.
If you can agree whilst you're alive on an option with the dc's dad AND the putative guardian(s), then yes, that is probably what will happen. No guarantees, though.
How likely are you & their father to perish unexpectedly together & within the next few years, though? RTAs aside, a far likelier scenario is one of you surviving & parenting as a widow, so I would focus on the financial ramifications of that.