So you will be five people altogether? It can sometimes be a bit tricky finding accommodations - usually the limit is four per room. However, we travel with our four children, and have always found solutions.
Since you'll be living in Ireland, would you consider taking a Ryanair flight from Dublin? That would open up some other options, including my personal favourite family destination, Lithuania. (We lived there for two years, I acknowledge I'm biased.)
Our four kids are a little younger than your sisters (our oldest is now 10), but we have had some great family trips recently.
In Czech Republic, we spent a week in Prague, which had lots of interest to both the kids and parents. We found a guest house in which we were able to rent two rooms for a reasonable price, and enjoyed the city immensely. One day my husband took the three older kids to the zoo, because none of them could take any more museums/churches/etc. and they thought it was excellent. I took the baby to more museums/churches/etc because she couldn't complain! Prague is easy to get around, you can buy a transit pass for a week which covers metro and buses, and there are lots of inexpensive places to eat. It's also a wonderful place to just wander around, there's are all kinds of hidden treasures. It does get pretty crowded, especially in summer, but we went in August and managed fine.
We also thoroughly enjoyed Budapest, where we stayed for a week. There we rented an apartment right across from the main train station/metro station, which was much cheaper than hotel rooms or a hostel (because we would have had to pay for 6 beds). We had intended to go out of town for some day trips, but we had such a great time looking around the city, we never got the chance. We all spent one afternoon at the zoo there - it's one of the oldest in Europe, I believe, and quite unusual. Even the younger kids enjoyed the museums of Applied Arts in both Prague and Budapest - they're not quite so stodgy as regular museums, and sometimes have fun activities for families. There is a lot of other information about museums and activities that are fun for kids in the "Family Travel" forum, be sure to check that out and see if anything appeals to you.
There are some cheap hotel chains throughout Europe (Etap hotels are our usual default), which are basic, but when you have to book two rooms, the price is right. We have stayed in hostels in a few different places, usually we end up getting a 6-bed dorm, but we still have to pay for each bed, so that can add up to more than a hotel/apartment in the end. In France, the Gites d'Etap are an excellent option. We just had a fabulous week in Normandy, mostly exploring the D-Day beaches, but also enjoying some of the beautiful cities there, and it was the most enjoyable holiday we've all taken together - even the three year old had fun and remembered stuff.
We have also had great family trips in Spain, France (Paris and Normandy), Germany (our current home), Italy (Rome/Naples) and the Netherlands (some great little towns in the east, not so touristy but really pleasant and with cool stuff to see), some of which are covered in the Family Travel forum as well, so be sure to ask more questions! Also, if you could give a more specific idea about the kids interests, I can ask my daughters what they specifically enjoyed about places, and that would probably be helpful to you.
Good luck with the planning!