This is an excellent answer, I would just like to clarify a point. The child can be shown in more than one family, by adding other parents. (Or you can do it the other way: select the other set of parents and then add the child and using Select Existing Individual.)
As Dave points out the relationship can be biological, adopted, etc. but cannot be a combination of relationships (even if that is the reality). By default the relationship is biological and if there is already a "biological" relationship the relationship to the next set of parents is "adopted". PAF will not allow a biological relationship to two different sets of parents, even if a biological parent has remarried and the child is shown in both families.