One of the best things you can do for your stepkids is to show kindness and love to their dad. They have likely seen a lot of negativity between their own parents through divorce or a breakup. Modeling for them a loving and kind relationship will do wonders for their future ability to enjoy and be a part of loving and kind relationships in their own lives.
One of the first things they will be watching for is the way you treat their father. They anticipate that they way you treat their dad is the way you'll treat them too. If you are loving and kind to dad, you have a much better chance of building a great relationship with them.
How do you show your step kids that you love their dad? How has it affected them to see their dad's new relationship?