The variety and complexity of family life today means that new skills are having to be learnt all the time. Step-parenting is one of those new skills. In this situation it's not only your feelings that have to be taken into account, but those of your partner and the children. You need your partner's support and understanding, and together you should agree guidelines on how to treat the children. Some of the areas that you should agree on are discipline, privacy (for all of you) and arrangements with the absent parent.
The children also need to know where they stand and what role their step-parent plays in their lives. There may be some resentment on their part and it could take them some time to come to terms with the new arrangements. This is a new relationship for all of you and if you don't have children of your own, a ready-made, part-time family will be difficult to deal with until new routines are established.
A Relate book that can help is Relate Guide To Step Families by Suzie Hayman
If as a new family you really are not all getting on Relate Family Counselling can help. Find your nearest Relate that offers Relate Family Counselling.