Dealing with remarriage questions

[Apathyisthenewblah]

I'm not a lone parent but am about to become a step mum to 2 lovely DSCs and thought the people in here may be able to advise me.

DP and his ex have been separated 5 years and we have been together 4.
DSC (to be) was just shy of 3 when they split up and it was instigated by DP. He now has a civil but not friendly relationship with his ex.

We are about to marry and although DSC has always seemed happy it is now clear that they are not.
They are asking some very difficult questions such as "how can daddy remarry when he couldn't keep those promises first time" and why does Daddy want to marry apathy and not mummy". DSC is also displaying a lot of anger to his mum about DP leaving although when he is here he is not like this. I guess he is acting out with the parent he feels most secure with.

I was hoping someone would have some advice on how DP can deal with this in an age appropriate way? I have no clue!

[GEM33]

Can anyone help? There must be some books about this situation ...