"...she is almost 6 weeks and I am getting hassled by my mother-in-law about the time she gets to spend with her... I am so protective that I have almost a meltdown when she wants her to stay with her at her house to visit"
Wow, 6 weeks *is* really, really young for your mother-in-law (MIL) to be expecting you to just leave your new baby girl! Those protective instincts you have are GOOD - honor them! Your baby has those same instincts - she knows your smell, your touch, and expects to be close to you as she was for the 9 months she was developing inside you. Did you know that the first 3 months of a baby's life are often called "the 4th trimester" because newborns still really, really need to be close to/with their mothers, that familiar heartbeat, their source of nutrition, safety. At this young age, your daughter has no idea if you're coming back when you leave her temporarily (mentally, she doesn't understand this - and won't for many more months), and this causes your daughter stress. So is it worth it for *both* you and your daughter to be really physically and emotionally stressed out just so MIL can have some time alone with her 6-week old granddaughter? I think not - just my opinion.
Would it work to just be gentle and honest with your MIL about how you're feeling? Would she understand if you said that as much as you trust her ability to lovingly care for your daughter, you feel that your baby is just too young for you to be away from her, but you're happy to stay around while your MIL and your baby visit with one another. Surely your MIL would have to respect this, that you're the mother and you must trust your own instincts, honor your own motherly feelings? If you feel that it doesn't work to be honest with her, you could always make up some excuse - if you're breastfeeding, you could just tell her that your daughter doesn't take a bottle, etc. If you think research evidence would help, you could tell her about this:
http://www.attachmentparenting.org/idealseparate.shtml
While I think it's wonderful that your little girl has a grandmother who is so interested in her - and while I also think it's great help your MIL get to spend time with her granddaughter - I think it's *most* important for you and your daughter to be with one another at this very early stage.
Congrats on your new daughter!