Millions of Americans target shoot. And the last time I checked, hammers and vice-grip pliers are sold in stores across the nation for perfectly ordinary uses. So why should the presence of targets and tools in a shared car indicate knowledge of criminal intent?
So, half a dozen Middle Eastern men visit a home where a Middle Eastern man lives, over the course of a week. What's to report? I read your link fully just in case there was something more than this, but didn't find it. If a White family had half a dozen White visitors over a week, or a Hispanic family had Hispanic visitors, or a Black family had Black visitors, is that suspicious? If it had been reported, would the police or the FBI have considered that to have met the evidence threshold for opening an investigation or even an inquiry, much less surveillance or a search warrant? Isn't it a bit racist, to assume that whenever Middle Eastern men congregate, terrorism may well be involved?
You write that the mother "belongs to a pro-caliphate Muslim faction". She belongs to the Islamic Circle of North America. From the Wiki page:
"ICNA strongly condemned the 2010 Times Square car bombing attempt and immediately called for punishment to the fullest extext of the law for anyone who engages in terrorism.  In 2011, ICNA welcomed President Barack
Obama's counter-terrorism initiative.  . . . ICNA responded to ADL's allegations by saying that its conferences have always been held under the objective of rejecting extremism. ICNA's statement also supported the defence of human rights for Jewish and Israeli people, but demanded the defence of human rights for Palestinians as well. "
So, it isn't exactly clear to me that the mother would have supported the views allegedly held by the son, much less the actions, had she known about them in advance.
Here's the quote from the father about the son:
"He said he shared the ideology of al-Baghdadi to create an Islamic state, and he was obsessed with Israel," the father told a reporter in an interview outside the home of his other son, Syed Raheel Farook, in Corona, California.
Assume that the mother also knew about these views, though it isn't clear that the ideology to create an Islamic state means anything more than that. (Is Saudi Arabia an Islamic state? Is Qatar, where CENTCOM has its overseas headquarters?) Does that mean that the mother was a co-conspirator? Does that mean she knew about criminal activity? Are we even sure that the attack was premeditated?