My brother and his wife are upset because I didn't invite her parents and siblings to my wedding. I have only met these people a handful of times but they say that "family" (even though they aren't really MY family), should come before close friends. I can't find anything about this topic on the web and was wondering if there is a "rule" for this situation. Is it expected that I invite my sibling's inlaws to my wedding? And now that I haven't, how do I patch up things between my brother and his wife and I?
No, you do not have to invite your brother's in-laws. There is no etiquette rule demanding this. Basically, you invite whomever you wish to invite. Yes, that should include family, an escort for each guest (spouses, significant others, etc.). And yet, you are the last word on who gets in the door--there is always a space issue.
Invite those to whom you feel closest and please do not feel guilty.