Normally I would say "Go, have fun, enjoy yourselves!" I sincerely believe 7 days in the life of an eight month old doesn't mean anything, will do no permanent harm, and could actually be good for her for lots of reasons.
BUT, if it's your Mom who has the problem with you leaving then there's no way in good conscience you can leave her with your Mom. Your Mom, consciously or unconsciously, may be trying to tell you she's not up for the challenge. And she might have some misgivings about the other caretaker. So deciding against the caretaker's wishes to leave her there is a terrible idea.
I do however think you should absolutely take your daughter to Spain. What an amazing trip for you as a family! I think all children, reagrdless of age, should be able to experience anything and everything cultural that they possibly can. This is a tremendous opportunity for you to expose your baby to new and exciting stimuli. Babies are usuallly pretty flexible and 8 months is still a fairly easy age for travel. Plus you'll get lots of positive attention in Europe.
As for your husband wanting a vacation from his new baby. Well, I kind of hate to say it, but it sounds as though he's not exactly accepted the idea of being a parent just yet and all that means in terms of compromise and sacrifice. If he needs breaks from time to time then you send him off with his sister on a big day trip and you walk the baby around a museum in a stroller. There are a zillion parks and places to walk in Spain and you can have a lovely time just you and your daughter. And if he wants to stay out late and go out at night then he can do that with his sister. Or perhaps she can line up a friend to baby-sit so you can go out too.
Relax your routines and go with the flow. By all means do NOT give up such an exciting adventure for all of you. And take lots of pictures. There's nothing so special as an adult than looking through an album and thinking, "Wow, my parents took me all the way to Spain when I was just a baby."
Have a great trip!
E.
Evanston