A: There's a failure to communicate here, and I'm not talking about between you and your mother-in-law. (Those words will be crystal clear: She'll say, "How about that sleepover?" and you'll say, "Look at the time. I'll call you later.") There's a misunderstanding between what your gut is telling you and how your brain is interpreting it.
Let's admit it: The "my daughter is too young" line is vaguely insulting to your mother-in-law, who could presumably handle a 7-month-old for a night. Tabling that excuse, ask yourself, "Am I worried about my daughter's reaction or my own?" In theory, if your baby is at Granny's house, you could enjoy a night out, plus eight hours of uninterrupted bliss. But maybe you're nervous that a sleep- over will disrupt your daughter's schedule, screwing up her sleep for weeks. Maybe she goes on feeding strikes or becomes clingy if her routine is altered. Or maybe you worry that her grandma isn't up on the latest safety info because she insists that her crib from 1982 is fine.
When you decipher what your gut is telling your brain, relay the message to your mother-in-law. Explanations as simple as "I don't think I could handle a night apart from the baby" or "I'm breastfeeding exclusively and I'm not ready to introduce a bottle yet" ring true when they are true. As long as you speak from your heart, there shouldn't be a miscommunication. Once you talk out the problem and explore other options (perhaps you and Grandma can swap houses so your baby stays in her own crib, or maybe you explain that you just want to wait until you're finished breastfeeding), then a sleepover is not only possible, it can be enjoyable for everyone.