Thank you for your question. I’m very glad you asked this because yours is a situation that many can relate to in this day and age. It is not simple to answer however.
I totally understand your point of view ; it seems you have raised him for all these years and you are the true mother in his life. As you were saying his birth mother wasn’t there for him for many years and didn’t want much to do with him. I completely understand that you feel offended by his gesture to include his biological mom on the invitation.
My question (and caution) to you is that by denying him this will it negatively impact your relationship with him? Traditionally speaking there are no right answers or appropriate etiquette for this type of situation in an Indian family. It is indeed a modern day circumstance. I think its important to remember that one can never know the impact of a biological parent – present or not – on a child.
It sounds like you have built a great relationship with your step son and you should be proud. If this is truly his wish and he is not being coerced by her, then arguing about this may jeopardize your relationship. All your good intentions over the years may be undone with this difference of opinion. Please realize that you will elevate yourself to a much higher and dignified position by including her name.
In my opinion, everyone close to you already knows what you have done for him and what you mean to him. This small gesture of including his birth mother’s name will make him happy and you will come out as the level headed parent that has raised him all these years. Looking at it from his angle, he is not seeing what she hasn’t done for him rather what she has and that is simply, given birth. To him this seems to be enough.
Another thought that may be of interest is to have his birth mother’s name in a separate section under the heading “With Best of Compliments” or under the “RSVP”. I hope this helps with your situation. Good luck with all the festivities!