First...Congratulations! I think in no way are you being a brat or silly or selfish!!! Every child is a blessing and a miracle and should be celebrated.
It seems you feel a little uncomfortable with the shower idea and asking someone to throw you one can be awkward...especially if they decline. I agree with the mother's suggestion that you and your husband host an event. Or perhaps you could host an event in honor of your husband since you mentioned he was not involved in celebrations for your other two. Maybe, you could host a "shower" for the two siblings as a way to honor them and prepare them for the new happy family addition. You can always instruct "no gifts, please" on the invite and then just give a few meaningful gifts that you purchase yourself.
If hosting an event is not possible or desirable, perhaps you can ask family and friends to participate by writing special notes or blessings to your new child that you can then put in a scrapbook for her/him. I did this as part of my baby shower and it is the most precious thing . Perhaps, you can have them send you scraps of fabric and have them made into a quilt for your child.
These are just a few suggestions. Good luck to you and congratulations, again.