To not include Mil in bday plans

[Unpaidcarer]

NC for this. DP has a milestone bday in 6 weeks. He has ongoing mental health issues. Mil has no interest in supporting him, despite my regular pleas for help.
She is in complete denial and blames me for his issues, despite them starting years before we met.
My Aibu is: our DCs have planned out DPs bday from morning to night. Intricate detail because they know DP needs this.
Just read on FB that SIL is planning on coming home for DPs bday and MIL has commented that she's looking forward to family being together for the weekend.
DCs are not DPs children. MIL has never showed an interest in them. No one has mentioned anything to me or DP about wanting to spend time with him over his bday. He has seen sil once in 4 years.
Do I
a) alter plans due to a fb thread I wasn't actually included in
b) pretend I haven't seen it and let dcs carry on their plans w the chance MIL and SIL may spoil it
c) call them now and tell them to back off as plans are set

I think you need to send a message to sil to say DP is not available to be seen/see her on his actual birthday and is unlikely to want to see her at all.

@perfectstorm the children are not aware of the more serious side (He was hospitalised after the breakdown and they were told he was working away)
He is in a much better place now but still poorly. We have been open about his depression as it's important they understand these things are part of life. MH issues are covered in school now.
I've spoken to DP today, just in a 'what would you like to do for your bday?' sort of way. He's quite determined to have the day with me and DCS. He'd like to see my parents on the day as well.
I mentioned MIL and he said 'if she mentions it I'm just going to be honest and say I'm busy on the day but I might come over the next day'
He was really chilled about so maybe I'm worried about nothing.

Well that sounds hopeful. Can the children factor in a visit from their nice gps as well, as he's said he'd like that?