Sending teenagers to Europe??

If it were my kids, I don't think I would have a problem with it. Your daughter will be well-experienced in being in Europe - dealing with language and cultural differences, knowing where to obtain services, where to stay, etc. And there is safety in numbers. If you don't have concerns about sibling stuff, then I'd say let him go. It will be an unforgettable bonding experience for all of them, particularly since they are approaching the ages where they will start pulling away from home anyway.

I think it's great that your older kids want to bring their little brother along and I predict they'll take superb care of him. Sounds like you have a very nice family.

I think it would be a great opportunity for your 15 year old son. I would suggest that you get an notarized paper giving your 21 yr old daughter permission to get medical treatment for him if necessary...never know when one might fall, food poison, etc. It's a 99% chance that it won't be needed, but you want them to have it just in case.

"I would suggest that you get an notarized paper giving your 21 yr old daughter permission to get medical treatment for him if necessary..." Ten years ago, yes, this would have been a good idea. But these days, with the ease of international telephone calls, these documents cause more confusion than they solve. In a true emergency, the universal standard is to treat until an authorized person (ie, you) says "stop". Anything less serious, and it's a simple phone call to get consent.

And I should offer the caveat I'm not sure what the consent laws are in Spain but they're pretty sensible and straight forward in Belgium, the Netherlands and Germany.

Our two sons did the same thing without any problems. When our son finished his semester in Madrid his younger brother, then 16 join him. Despite their six years difference they have always had a strong relationship and it work well. If everything is as you described, then I would have no objection to his going. You MIGHT have another issue these of an unaccompanied minor traveling alone. Or would both the 18 and 15 travel together. In this world of changing security rules and regulations you should double check to make sure that no one has decided that a 15 year could be a terrorist.

As a reward for scoring high on her AP Spanish exam, we let our 15 year old spend the summer near Madrid with our former exchange student and family. The parents were working most of the time, so she was with her 21 year old Spanish "sister" daily. The two of them also did traveling to other parts of Spain on their own. (My daughter brought her "Rrrreek Esteves" guide book along, which is what they used all the time in Spain to pick hotels, etc!) I was totally fine letting her, at age 15, travel with a 21 year old. The older girl was very protective of my daughter as far as friends smoking and drinking around her. My daughter is very sensible and mature for her age, so I never worried for a minute. How wonderful that you have 4 kids who get along so well and want to include the youngest in the adventure. You obviously did a lot of things right and turned out some great kids. I hope you let them all have unforgettable times together that they will always cherish. These kinds of opportunities for the 4 of them to be together are very limited as they grow up.

Tom, international calls were great even ten years ago. I had a friend that her grandson went over to the UK last year (15 yrs old), his parents sent an emergency medical form with the friend giving them permission to get him treated. Well, he got food poison and needed medical attention, the drs., told them it was great that they had the form and it saved a lot of time.

In this day and age, I'd spring for the few extra dollars to get the notarized form making your eldest his guardian. You'd hate for him to get stopped at immigration and refused entry because he's a minor.

"the drs., told them it was great that they had the form and it saved a lot of time."

Glad it worked that time, but I've seen enough of these poorly worded consent forms delay care to conclude that it's just better to call home if consent is needed.

Just a note Karen, if you go for the notarized form and have a CPA, financial advisor or insurance broker that you've used regularly, check to see if they have a notary on staff who will do it for free. I am a notary and we never charge our clients and neither does my insurance broker or financial advisor. Also, here is a link to a form I have used when traveling with my kids withouth my husband. Luckily I have never had to use it, but at least it will give you a starting point.
http://www.singleparenttravel.net/wp-content/uploads/2009/06/Minor-Consent-Letter.pdf