I don't know if anyone of you have older children but would you consider putting one of them as guardian for your youngest in your will? That's the question we're facing now-with a new one on the way our wills will have to be updated.
Seems like a really sensible idea, presuming that the older child is able and willing of course. I would also want to make sure the financial aspect was all taken care of.
The finances will not be a problem. Just to be clear--this will be a half-sibling who is already grown so won't be 'growing up' with their younger sibling.