Peace Bint-e-Naeem
People ask Mufti's to provide criteria for allowance and that is in my understanding the wrong approach to seeking advice. Rather we should look to what the essence of noble conduct is in Islam, rather than asking if something is allowed we should ask if something is 'good' or considered 'excellent' to do.
The only case where the question "Is it allowed?" should surface is when there is no option otherwise or if by not doing so we go against a law in the country we are in.
In such a case of "baby showers" look to the essential intent of the act. Is it not just another excuse of promoting other to give you gifts?
So one element is that it would most certainly be better to throw a party for someone rather than throw a party for yourself, because then one cannot be blamed for trying to get gifts from others - so this way one cannot be blamed for being materially driven.
You say the intent is a bunch of ladies gathering, eating, chatting and playing games.
This again is fine but a better position would be that when gathered to share with each other hadith and recite Qur'an. Speak less and avoid gossip, eating healthy food and conservative amounts, playing games but not during prayer times, and so on. This intent so long as all who attend know this will turn a relatively neutral gathering to one that may have a lot of blessing.
Lastly, which is the reason why I believe that "baby showers" although may be allowed are not entirely 'noble' acts, is because the best of people in our Traditional Islamic past women have tried to stay in private away from others in times of pregnancies. The manner in which Maryam (AS) stayed away from people reflects her piety. Finding references of women pregnant is difficult for may be two reason one that the narrators were too shy and the other reason is that those women more than likely avoided the public. This reflects the behaviour of people rather than a legal ruling of what is allowed and that is probably the reason why your sister has reservations regarding this modern practice.
I totally respect some of the things you mentioned in the start. But what do you do when people start playing the Halal Haraam game? I personally don't find myself in the position of declaring what's 'jaez" & "najaez" in Islam since I have a lot to learn. I know we should be using our common sense too as well as follow the Quran & Sunnah. But how do you respond to people then? Not saying its a debate here. I find it to be a form of learning more about our religion. But how do handle people who instantly call you a "gunnahgaar" without even thinking twice.
From your post I gather that ladies back in the days didn't expose themselves much while being pregnant. But does that mean hiding one's self from the ladies in the family or their girl friends as well? Most of the times expectant moms share the news with other moms or ladies so they can share the happiness & excitement & also to get advice & tips on coping with the whole situation.
I know that its recommended to hold the news & tell everyone once the baby is born in the form of Aqeeqa. I thought that was for the outer world. Not your immediate family or friends. But I also always used to think its our Desi "culture" that frowns upon women who don't do an extra effort to hide their bellies.
Going back to baby shower. The argument came up because I have a cousin sister in law in my home city who is most of the times ignored by other ladies in the family ( call it family politics) I thought it would be really nice if all the ladies & girls in the family surprised her. Its creates good will as well as everyone has a good time.
You mentioned sharing hadiths & reading Quran. Although I love the idea of sharing Hadiths (probably make Islamic games & quizzes as well), I also know that its never a good idea to ask people to read Quran in a gathering. I have seen numerous people disrespecting the whole idea.
I can also keep in mind nothing collides with the Namaz timing. Jazak Allah Khair for mentioning that as well.
But then as you mentioned in your last paragraph. Does that mean baby shower is not considered a noble thing in Islam. What about American Muslims? As I find it to be a tradition strictly carried among women over here.
p.s I am not trying to contradict you or make sure that I get a positive answer to this question. Just want to make sure if what I plan on doing is right or wrong & will I be getting any gunnah since I am the one who is initiating the whole thing.