All babies are unique and while now you are doing it tough in a few years you might sail through the teenage years and your SIL might do it tough then.
Also get out of the house baby free, start with just a small parcel of time. Your DH and DD will be fine without you for an hour. Your DH will work out strategies to settle and soothe your DD if your not there (and while they are working out what works for them you won't hear it).
I used to pop down to my BFF's house between feeds for a cup of tea. Being able to drink a whole cup of tea, uninterrupted while it's still hot was bliss in those sleep deprived days. It was very good for my mental well being and a good bonding opportunity for DH and DS.