This is an excellent question and one that I think about often - try not to stress about - not easy.
I rewrote my will after my divorce and named my son, Brandon (age 28) as Ryan's guardian. Brandon told me at age 19 that this was what he wanted - he NEVER wanted Chuck and maggot to have Ryan. So that is how it was written and also that Brandon would look after Ashley and that they could all remain in the family home. Of course, several years later - things have changed somewhat. It still think that Ryan's siblings could make joint decisions about his care but despite what you write in your will - anyone could contest and a father would likely have a strong case here. Not much I can do to totally prevent this but I'm sure Brandon and Ashley would bring up many past behaviours that might hamper his getting Ryan. It would not be a good scene - that's for sure but I've done what I can to try to prevent it.