What to bring? Mostly just yourself, I guess. All that the new mothers in my wider family ever seemed to need was a maid for the household chores and a nanny for any older kids. I'd say you've got that covered just by visiting. Maybe screen for anything your Portland sister doesn't have that you found useful for nannying your Boston sister's kids?
Is your nephew old enough for a bike tour yet? Or if not, does your sister have a bike with a child seat? June seems like a good time for biking, and Portland is one of the few American cities that treats bikers as first class citizens of its roads.
About the present: Are you looking for something useful to prevent stress, or something fun to diffuse it? A good stress preventer, if your sister doesn't already have it, is a baby carrier backpack. You put one kid into it, carry it around with you, and have your hands free for the other.
I'll be in Portland visiting Blatham this coming weekend. If you'd like me to take a look at any particular place there, I'd be happy to.