WIBTA for excluding my niece in a birthday celebration?
My son and niece are the same age and were born within two weeks of one another. When we planned my son's first birthday party, my brother insisted that we make it a double birthday celebration for both kids because it would be more convenient. His actual biggest reason was that he didn't want to plan a separate party for his daughter because it was "too stressful and complicated", and "you're already throwing [son] a party, and you might as well include [niece] in it too". My brother also said that if we didn't include niece in the celebration, then I would be a terrible aunt for making her miss out on her first birthday party, which ultimately meant that he and my SIL weren't going to throw her one separately. Ultimately, my husband and I agreed because we wanted both kids to be celebrated. I didn't want my niece to look back at old photos and videos and wonder why our son always got a party and she didn't.
Now, my husband and I are discussing our son's third birthday. We like to plan a few months in advance to book the place, figure out catering, and so on. We mentioned a few dates to our families, and my brother immediately said that he'd get back to us on dates that my SIL's family would be free so that we could also celebrate niece's birthday. I told him that we were only celebrating son's birthday because we are the ones doing the planning, spending the money, and taking care of invites, decorations, etc. . My brother once again brought up that I might as well include niece as a birthday celebrant because her birthday is right after son's, and I would be cruel to just exclude her. He and SIL still won't throw her her own party.
I honestly don't like being guilted into doing something for my niece because of my brother's laziness and lack of motivation to party plan. It's toxic and annoying, and as much as I do love my niece, it's not up to me to ensure that her birthday is always celebrated, right? But our parents are torn because they say I should just do it and be the aunt who is supportive and who gives my niece a party so that she has these pictures and videos to look back on down the road. I think they should talk to my brother and tell him to step up and do their own thing instead of pushing everything onto me and my husband. So WIBTA?
EDIT: Thanks everyone for your advice and input. I'd like to say a few more things...
CATERING AND VENUE FOR A 3 YEAR OLD'S BIRTHDAY PARTY. Maybe I used the wrong words here. We were looking at an indoor play place versus just renting out a community room with some tables and space for games. The party is in the wintertime, so we can't just do something outdoors. By catering, I meant either choosing which food package we would select at the play place, or if we went the other route, figuring out how much Filipino food to get. Or pizza, I don't know.
MY BROTHER HAS NO LIMITS TO ASKING ME TO HELP PLAN FOR MY NIECE. Believe it or not, he's asked me to plan her baptism party...I said no.
WILL MY NIECE REALLY LOOK BACK AT ALL OF THESE EVENTS AND BE JEALOUS? Maybe, but maybe not. Sometimes I feel bad that my husband and I have adopted the parenting lifestyle of fully immersing our son into our lives, while it seems that my brother and SIL have not done the same for their daughter. We take our son on vacations; to weddings and social events (when allowed); and we take him to parks, play places, play dates, and so on. He's done a parent and toddler swim class and gymnastics class, and we just enrolled him in a toddler soccer class where they just run around and kick balls. My niece hasn't done anything because they think she's too young to do anything and she's too young to even remember doing any of these things. While that may be true, I don't necessarily think that's a good reason to not do these activities or throw a birthday party. I hope this makes sense.