Actually I took my 11 yr old on a Family RS tour .. their were 14 children between the ages of 10-17 .. ( actually of that there was on 7 yr old and one 18 yr old.. but the bulk were 12-15 I would say and the older girls did not seem to mind one little bit.. they all got along great..
But.. I would also ask you to consider doing this on your own.
You can and will save money and can tailor what you see to your kids interests. I did this with my 13 yr old son as he was not a good tour candidate ( he had /has some issues).
I made it easy.. flew into Paris.. spent about 10 days there.. then took Eurostar to London and spent another week there.. flew home from London.
You could make it easy.. fly into say.. Amsterdam, few days there.. train to Paris.. week there.. then either fly to Rome for a few days/week, see Venice.. or go over to UK on Eurostar and visit London, and maybe train to Scotland.. the choices are literally overflowing.
Your girls are big.. much older then mine when I planned our trips.. and I made each child do some research and come up with what they most wanted to see.. and to list three must see sites in each place.. if an 11 yr old can do that.. I am sure your kids can.. its sweat equitity for getting a super family vacation.
I would start with talking to girls and asking each for their top three destinations ( how long can you be away.. since I am not one that rushes from place to place myself.. and would say 2 weeks equals no more then three major destinations.. from which you can also plan many daytrips ) .