Sometimes a researcher is tempted to "ignore" the "in-laws" or "step" relatives because they are not "really relatives." However, this can be a big mistake. Your relatives interacted quite a bit with these individuals and there is a chance a record on them could provide information on your ancestor. It is always possible that these indivduals have known your ancestor long before they became related by marriage.
Your ancestors did not live in complete isolation. Researching their close acquaintances may provide information on your direct line ancestors.