Brief BG:
Ex and I have been divorced for 6 years. We have one DS together, who is 11. He splits his time equally between Ex and me per the custody agreement. We are on friendly terms, and we also work hard to make sure that we are respectful of our custody agreement.
DH and I have been married for 3 years. He has 3 DSs. All are independent and living in other cities.
My brother lives 2 hours away. He is divorced with 4 kids. They were separated a 18 months ago and finalized a few months ago.
My mother lives 30 minutes away. We don't see her often because she is manipulative and difficult. My dad died 7 years ago.
For many years (I'd estimate 15-20), holidays at my mother's house have been organized for the convenience of my brother and his family. My mother and brother discuss what will work for him and I am informed. Then I am expected to find a way to make it work for my family regardless. Over the years, I began feeling increasingly resentful. I spoke with both of them about this a number of years ago to no avail. They see no reason to plan differently. Therefore, I concluded that because I cannot change the way they handle holidays, I have to change the way I respond. I know I don't have to accommodate them. I started saying no.
My mother doesn't respect my custody agreement with Ex. Over the years, she has circumvented Ex's weekends with DS when my brother comes in to visit. If I say no, don't bother Ex, she will call him anyway and manipulate Ex into giving up his time with DS. One Christmas several years ago, this resulted in an argument between my mother and me, and I limited contact with her severely after that because she could not respect my boundaries. I have increased contact with her slowly, but it is still limited. Her M.O. is to lie and manipulate others to get what she wants.
End BG.
Easter is upon us. My mother e-mailed me a couple of weeks ago to inform me that my brother and his kids won't be there for Easter because his Ex has them that weekend. She also informed me that she invited some old friends of hers for Easter. I was informed that we could come on Easter to eat with them. She and my brother also decided to reschedule the family Easter for the 15th, and I was informed of this as well.
DH's birthday is on Easter Sunday. He decided that he wants to spend the day with his DSs, who will be in town this weekend for the holiday. He would prefer not to spend his birthday with my mother's friends. He missed out on seeing his DSs at Thanksgiving because my mother again managed to create a large scheduling problem that involved DS. I don't intend to let that happen again.
Unfortunately, DH and I have DS on Easter and not on the weekend of the 15th. As you've probably guessed by now, despite this problem, my mother expects us to come on the 15th because my brother will be there. I contacted both my mother and my brother after I received her e-mail. I told my mother about our plans for Easter and DH's birthday. I told both my mother and brother that the 15th wasn't going to work for us because we don't have DS the weekend of the 15th. My brother said that he'd talk to his Ex and see if they could switch things around for the weekend of the 21st. Of course, he has not gotten back to me on that. Instead, I got another e-mail from my mother in which I was informed that she and my brother confirmed the 15th, and if that was a problem with DS's schedule, I should invite my Ex to dinner so DS can come. (Not surprisingly, this expectation does not apply to my brother with regard to his Ex.)
I should add that my mother never cared much for Ex. He's a nice guy and a terrific dad, but she didn't think he was good enough for me. She rarely spoke to him for the 15 years we were together. She isn't inviting him to dinner because she wants HIM there. She is inviting him as a ruse to get DS there.
So, here we are. Evidently, I am supposed to invite my Ex to the belated Easter dinner on the 15th at my mother's house so that DS can be there when it's convenient for my brother and his kids to be there.

As I see it, I have four options:
1) Decline the invitation for the 15th. DH and I would prefer not to go without DS. Time at my mother's house is really that unpleasant. We don't want to disrupt Ex's weekend with DS either. (Note: Historically, my mother would call Ex anyway and manipulate him to give up DS for the weekend if I say no.)
2) Accept the invitation for just DH and me. We see my brother and his kids; DS keeps his weekend with Ex. Unless my mother calls Ex herself.
3) Talk to Ex. Invite him to dinner. Then all of us (DH, Ex, DS, and me) could be at dinner on the 15th. This is my mother's perfect scenario. I dislike participating in the manipulative ruse to get DS to her house.
4) Tell my mother to invite Ex to dinner. She is the hostess, so I'm not sure why the onus is on me to invite him to dinner at her house. (Note: Am I being hypocritical with respect to the other options above? Is it better if I sanction her invitation?) If Ex says yes, then we will all be there. I still dislike the manipulative ruse of inviting him to dinner, but it's his decision.
I don't know what to do here. I'm leaning toward #4, but I don't want Ex to give up his weekend with DS because of my mother. I prefer #1 because DH and I would much rather not go at all. I've already told DS that we won't be going to my mother's on Easter and explained about DH's birthday, so he's not expecting to see his grandmother that day. He doesn't know about the 15th at all. I haven't said anything to Ex about the 15th yet, but I could put a bug in his ear.
Am I missing any other options?