Just because a lot of people are doing something doesn't make it less "poor taste." It just means that people nowadays seem to think it's okay to throw parties for every little thing, thus burdening others with outfitting their new babies etc. I don't care if it's called a "sprinkle," the purpose of a SHOWER is to "shower" the expectant mom with gifts. There is no other reason. A shower/sprinkle is a plea for gifts, plain and simple. It goes back to the idea that a young, new mother would need help setting up for her first baby, just as a young bride would need help setting up her house. When you are older and more established, you should not need help.
Just because you're having a boy it's "more appropriate?" So if a person is having another girl it's NOT appropriate to ask for gifts, but if they have a boy, then it's okay to ask your friends and family to outfit this child because then you don't have to go out and buy all new stuff?
Obviously you are uncomfortable with the idea, or you wouldn't be asking. And for good reason.
Miss Manners still says it's not appropriate "GENTLE READER -- That your daughter is right: Begging for gifts is normal these days. It is also vulgar, of course. You are also right that baby showers are supposed to be for the expectation of a baby's appearing in a household not already over-run with baby equipment.
But Miss Manners makes an exception for an informal gathering of the expectant mother's close friends who are moved to make a fuss over her a second -- or fifth -- time. However, the plea that a more formal gathering for the lady's entire acquaintance, complete with those detestable gift registries, would enable the guest of honor to parcel out her shopping is not charming."
I think a more appropriate thing is to throw a "meet the baby" party. Then the celebration is not built around the expectation of gifts, but rather the celebration of the arrival of the child. Do not register, but let people know what you need if asked. Your sister could throw this party, as you will be taking care of the new baby.
Or, as Miss Manners suggests, a very small, informal gathering of your closest friends, who are dying to shower you with gifts is also okay.
(Also, I'm keeling over with horror at the thought of someone throwing me a shower because I needed boy things and imposing that on my family and friends! If I can't afford to outfit or provide for my child, then I would not have one!)