Mke, a couple of suggestions.
First, bite the bullet and just plan on doing laundry both on the ship and during your travels. Staying with family will make that easier.
Second, a sports coat may also come in handy in Europe; take in on plane with you if you are concerned about packing.
Third, don't take "cruise only" clothes. I have found same clothes worked fine for traveling through Europe as on Med cruises; of course, I don't take the beach excursions in Med.
We have easily been able to travel for a week in a rolling 22" suitcase each. When traveling on trains in Europe, I really don't like to take a larger suitcase. If you want to take a 26" you will always find room to store on train; just harder to handle up and down stairs.
Trip sounds wonderful; have fun!!!