Going Back to Work Nervous Momma!

I want to start by saying that I'm so grateful that we have family members that are going to babysit for my little girl when I go back to work this week. 
How do I approach them about my wishes? Do I just give them her schedule and hope for the best? My biggest fear is naps. I've worked hard to get my daughter to be a good sleeper. She gets put down for all naps in her crib and at 3 months can self soothe herself to sleep and sleeps through the night. Problem is my MIL is taking a few days a week and she doesn't put her down for naps. She rocks and holds the whole time unless I'm there asking her to put her down. She doesn't sleep as long being held, and when she does put her down she is in there messing with her and talking to her so she doesn't fall asleep. When they babysit for me she is always overtired and very fussy by the end. I just dont want a fussy baby when I get home from work for the few hours a day that I will get to see her. I feel like I'm being crazy but I'm a FTM and I'm having a hard time going back to work.