Forgive me if this is lengthy, but hoping I have some thoughts that could help you.

My current husband's children were grown and on their own - but he inherited my sons when we got together. My sons lived with their dad, but spent lots of time with us.

Here is info about what we went through and how he built relationships with each of them.

Reality: It will be rough at first. Your fiance's kids will say nasty things about you to their mom, their dad and even to you. Be patient - very patient. (And when you feel very hurt and want to cry, do it in your fiance's arms in private).

Don't try to be a parent to them right now; do work at being an adult friend. All 4 of you need to spend time together doing "family" things. Go out and have fun together; cook a meal together; watch movies together, etc. This helps build their trust in you and also allows them to see and experience the growth of a "new" family unit.

Eventually (might take months), find one on one time with each child during which you listen to them, share a common interest, etc.

If they ever tell you a secret - keep it secret! Don't share their secret with anyone. This is part of their testing whether they can trust you. (Of course, if the secret could cause danger to them, share it quietly with your fiance so the 2 of you can develop a plan on how to handle it). But he will have to be the one who handles it.

While you both might want to get married within a year from now, consider waiting even longer. My hub and I lived together for 4 years before we married. My sons saw the commitment we had to each other and learned that my hub wasn't trying to replace their own dad.

This all happened in 1998. Today my sons refer to my hub as their father. Their dad is their dad - but my hub is the one (they have told us!) who provided the best guidance and is always willing to listen to them and provide support and advice.

You can make this successful - just be willing to give it lots of time and be patient.