I too agree that sitting through a shower when way preggo was awkward and uncomfortable.
One thing I enjoyed was a "meet the baby" get-together my family held for us about 6 weeks after our son was born. Not only did I have more support (extra hands to hold baby), it was inclusive of my husband and didn't feel like a "gift haul". And there were none of the silly shower games which, in my pregnant state, would have really annoyed me.
One other thing I noticed was that there was a lot of social activity as we geared up for our son's birth, but the days at home following his arrival seemed like a long stretch. As with many moms, it was nice to have something to look forward to--and a reason to dress up a little--after baby arrived. We love to pamper round, expectant mothers, but tired new moms often appreciate being doted on a bit, esp. after the days of nursing, caregiving and sleep deprivation that go with parenting a newborn. After all the excitement is over, it's easy to feel a little forgotten.
So, not to steal your original thought, but all this to suggest that maybe early April would be good? This timing also avoids the onslaught of all the newborn clothes, which the baby grows out of way too quickly, and will give your sis and her family a chance to figure out what they are really needing.