Married 3/04
DS 8/07
DD born 8/11
Not baby clothes, but I did borrow and return toddler clothes. That was really clear because I borrowed them between use of two existing children in the household they came from. I also asked about all HMDs whether they needed returned and would have been happy to return anything that the original owner indicated. This is all pretty local though and I'd be less inclined to take a loan that involved a long car ride or shipping. I think those that suggested watching for good condition, reasonably priced used items would be best. The expense of getting the items back plus the potential strained relationship if your SIL is offended by you wanting things back now or you don't get back what you expect (stained, broken, lost, etc.) just isn't worth it for used baby items. I get that they are your used baby items, but unless it's irreplaceable try to let it go. A gently used rock n play will be just as good as your gently used rock n play.
momma to DD 12/08 & DS 3/13
I agree with all the PPs who state that hand-me-downs, unless explicitly stated by the donor, are considered for keeps, or at least until the recipient is done having kids. I lent a TON of stuff to friends after having DS2 as I wasn't sure if we'd have another kiddo and we needed to clear stuff out of the house. When I got pregnant with DC3 it never crossed my mind to ask for the stuff back as I just assumed they were either still using it or had given it to other friends/Goodwill.
Also, consider that as it's your SIL's first baby, she may well want new stuff. With DC3 I am totally happy to buy almost everything used - near-new swings go for $25-30 on my local Craigslist, and as she can't wear many of her brothers' hand-me-downs, many of baby's clothes come from eBay or FB resale groups - but with DS1 I really wanted the box-fresh stuff (and luckily people are more generous with your first child so we were gifted the stroller, changing table etc). The stuff that you want Alice to return would be on its 3rd use by the time Jane got her hands on it, and may not be in great shape. Those 'secret milk' stains are a total PITA to get out - a friend gave me a box of her DD's old clothes when DC3 was born and I was too embarrassed to tell her that most of them were so badly stained as to be totally unwearable (I'm sure they weren't stained before she put them in storage 5 years ago). I would buy Jane a couple of lovely items from her registry and be glad to have all that baby clutter out of your home!