It happens to everyone--an urgent late night call telling that a loved one is sick and in need of your care. Being thousands of miles away and several time zones apart can really put the strain on you as you try to figure out a way to get home.
First, plan ahead. If you are negotiating with a company to be sent to the US, have it written in your expat package (if possible) that the company will purchase X number of tickets per year for emergency repatriation to care for a loved one. You can haggle on the terms, number of tickets, etc., but if you get a chance, get the company on record as saying they'll pay.
Second, if you can't get a company to pay for it, try to budget almost $1,000/$1,500 per person for an emergency return ticket to the US. Put this money in a savings account, or keep this available on a credit card (if you can get one). Last minute fares are generally more expensive than those you buy with several weeks notice, but you can generally find a last minute fare that is under $1,500 from most major airports. You can also try sites like http://www.priceline.com and 'name your fare' as there are almost always extra seats on the transAtlantic flights.
There are what is known as emergency or bereavement fares for such situations, but a) they cost almost as much as a last minute fare and b) they will require paperwork from the hospital, something you simply might not be able to produce in an expedited fashion. It sounds cold, but these fares were subject to much abuse before the paperwork requirements were instituted. The one advantage of these fares is that they are generally very flexible, and you don't have to leave on a specific date/time but have can return to the US when you are done with caring for your loved one.
Important: For this reason it is also essential that you ensure that you have the right travel documents to leave and re-enter both the United States and other countries and that these documents are always up to date. In particular, if you are applying for Adjustment of Status (AOS) you must make sure that if you need Advance Parole, it is up to date. Otherwise leaving the United States will mean your AOS application is abandoned.