SOME IDIOMS ABOUT RELATIONSHIPS 4º ESO

know someone inside out	If you know someone inside out, you know them very well. Sue and Anne have been friends since childhood.  They know each other inside out.
speak the same language	If two or more people speak the same language, they have similar opinions or ideas, so they understand each other very well. We work well together because we speak the same language.
run with the hare and hunt with the hounds	If you run with the hare and hunt with the hounds, you want to stay on friendly terms with both sides in a quarrel. Bob always wants to keep everyone happy, but he can't run with the hare and  hunt with the hounds this time - the issue is too important.  they're like cat and dog = to often argue with someone: "Those two are like cat and dog." And a lot more in the following link http://www.idiomconnection.com/relationship.html