There isn't really a good way to answer this using objectivity. Any image merge in this regard can be debatable, but some merges would be more obvious than others. The bottom line is that we prefer all images have relevance to this site, and so the less relevant the image is, the more likely it is to be merged or deleted.
The factors to be considered are:
There is no rule against having family in the photo, and so there isn't much of a reason to crop them out. But I would avoid uploading such images even if you do decide to crop them out because again, we prefer images that have more relevance to the site. Some users like to upload anything and everything of certain women just because "the image exists, therefore I must upload it." We don't want that.
Cropping should be done carefully. One thing I've noticed in the past years is a tendency for some uploaders to crop off pay site watermarks so they can upload those pictures.
If I see a posted image that could do with a crop (eg\; removing husbands/partners/children out of a group shot) and obviously saving without noticeable loss of quality (I know the debate about saving as PNG etc ) Is this permitted to potentially replace an existing image?