My second girl was born 10 yrs after my first, so I had to pretty much start over. I then had my 3rd (a boy) when my 2nd was only 15 mo old... so we needed doubles of everything, since my 15 mo old was still in a crib & in my veiw was still a "baby" eventhough she was starting to walk. My 4th was born 2 yrs later... with him I needed a new paypen, a few new bed sheets (since mine had gone through 2 kids already & were starting to get holes in them), and a new high chair/booster (since my other 2 were still using theirs). Other then that I asked for diapers (size 1 or larger), baby laundry soap, bath products (I loved the calming lavander), baby wipes, new nipples for the bottles (eventhough I mainly breastfeed - it's nice to have a bottle around for daddy to use if momma got stuck in traffic), new binkis if she uses them w/ the kids, and anything else she might use to take care of the baby on a daily basis. He also got a cute little blanket bear (the head of a bear w/ a blanket for the body) - he slept with it for the first year almost 2, he did get a replacement at his 1st Christmas just incase, because we couldn't get him to sleep without it.
I'm carring #5 right now... I feel as if I'm starting over w/ almost nothing this time. Both the cribs I have were on the latest recall list, the infant carseat I had stopped latching correctly when my 2 1/2 yr old was about 19# (it lasted through 4 kids - 3 of mine & 1 of my sisters), the bassinet is on it's last leg (it lasted through the same 4 kids) - although I do have TONS of baby clothes... I have been going through them all and will be taking a lot to the Goodwill or Salvation Army. I am hoping to get a few new things for the baby... some new crisp sheets, unstained burp rags, an outfit or two (it's nice for everyone to have something new not all hand-me-downs), a special blanket, a binki just incase (I've had 2 that like them & 2 that wouldn't take one), new nipples for my Aven bottles, diapers, bath products, laundry supplies, reusable nursing pads (my sister is making me some - she sells green baby products), and I also need a new nursing night gown.
Altought I don't need much I did set-up a registary at Wal-mart & Target with stuff I'd like to have for the new baby... if your sister-in-law has time, maybe she can do this. That way people know what she thinks she need or the "theme" for the new baby... mine has kinda changed with each one.
Good luck finding the right gift & congratz on the new little one coming into your life through your brother & sister-in-law.