If you guys are having a birthday party for SD, why does your DH want to go to BM's? It works for some people, but not for others and that is okay. When a BM is high conflict (and wants your DH back) I think it's best to just stay away as much as possible. I mean, that's just the reality of divorce.

I would explain that to your husband, plus the fact the BM is so disrespectful to you, his wife, he just should not condone that behavior. And, you are probably right, she has some reason for this. Has she done this before?

Boundaries need to be set and stuck to. Or, all hell breaks loose.

Ask your husband if he understands to whom he is currently married. You have already planned a party for SD so there is absolutely no reason to go to bm's. 

Personally, I think that she invited you both in the hope you'd invite her to your party, thus giving her a foot in the door. 

If your husband accuses you of selfishness, ask him how it's being selfish not wanting to be in the home of a woman who he used to f**k. Tell him that she is his EX and of no interest to you. She has no business in your life. Ask him why he can't let go... 

Don't let him bully you into going... 

I bet BM wants to see the baby. Her exMIL texts her pictures of your child? And BM has a history of wanting to get back with DH? Sounds like she is jealous and dying of curiousity since you and DH had a baby. And she's using the birthday party as an excuse/cover up. I'm assuming BM has never suggested joint birthdays before... it's the only explanation I can think of. 

And no I definitely wouldn't go. I wouldn't want BM to have any access to my child at all either. Your DH is clueless if he thinks it's "just a birthday party." HCBMs have alternate agendas. 

Or, introduce BM as Cruella or the wicked witch, or as a monster to the baby when she comes up to try to inject herself with the baby.  No need for the baby not to know that BM is the Wicked Witch.

It would be different if the BM was a sane, normal person  who co parented well with you and your H. If you all had a cordial relationship, then maybe you MIGHT consider going (only might, not definitely).

I know a few people who do this, but on the whole they are all happy, have moved on, have settled into new relationships and keep everything civil for the kids sakes. It's not that common.

In your situation it'd be a hard NO from me. BM obviously has an agenda. Your H needs to pull his head out of the sand.

Also, I would be very very angry if my MIL shared photos of my kids without permission!

You need to pull her up on that.

DH and his mommy need to separate themselves from BM... BM no longer should be getting any pictures of your child, should not be left alone with your child and needs to be blocked on all social media until she can get her act straight.... DH and you need to go to counseling until he can learn that he needs to be on your side.... y'all are supposed to be ONE unit and your side is the ONLY side he should be on

Have your own that categorically excludes BM.

She can have hers, you and DH have yours.

Eeeek! This is stuff of nightmares. Don't go, and keep your baby away from her! I would be furious if my MIL was sending Toxic BM pics of my baby. That's just boundaries not respected at all. Your DH needs to seriously start stopping that BS.
 

I made the mistake of SD holding my baby when she was FaceTiming Toxic BM for her birthday, and then Toxic BM brought in her BF's baby and was competing with that baby with mine! Ugh! Cringe! Thank god my baby wiggled out of SD's arms and wasn't interested, and she is only 1 and will never remember it. I won't let that happen again. I told my DH from now on she won't ever lay eyes on our baby. Ever. Or our dog she mistreated when she was with DH. It's just none of her business. 

So I had a very similar situation with our crazy BM.... many times trying to get back with my DH.... after years she realized DH was blocking her and not dealing with her once the children were grown.  She was always crazy jeolous and nasty to me.  Then ALL of he sudden changed and invited us over after SS graduation like we were best friends..  DH thought she was turning over a new leaf.  UMMMMM NO that is where men are dumb.  She was now trying to be friendly while still back stabbing just to be around DH.   Figured she couldn't be withouth me or while being nasty to me so, this was her next game.

Game it was too!!!  Stay out of it.!!  and your DH should too.  If she was trying to get him back before  it is still her end game.  Sure friendships can work but not typically when they start out with boundry breakers!!!  Telll your DH you will have your own party.   

 the Spermidiot has never moved past being dumped by my DW when she had enough of his cheating, etc...

They were never married so there was little drama once she booted his ass.  For the first few years after we married he would occassionally call with the tearful "I miss my family and want you back" crap.

DW would laugh at him, ask him if he wanted to speak to his kid then hand the phone to then toddler SS or ... just hang up on him if SS was playing.  Most pathetic, non one person in the SpermClan has ever called to just talk with the kid. Not once in the 16+ years we lived under SS's Custody/Visitation/Support order. Assholes.

After the first few years it shifted to periodically whinning to SS "Does your mom still love me?" during SS's SpermLand visitations. Even when he was fairly young SS would laugh at him, comment that his mom and dad had been married for a long time and love each other very much.  SS has always called me Dad(dy). I was the first person he ever called Dad(dy).  The Spermidiot was always "Daddy (Firstname)" when SS was young. As an adult, my kid calls me Dad and refers to the dipshitiot at "Gangster Dad" though not to his face. SS just calls him by his first name.  His work friends know who the kid is talking about and get all twisted if the kid says something about "Gangster Dad" without using the "Gangster" prefix.  "Ghasp, your Dad would never do that. You must be talking about Gangster Dad!"

Interestingly, after we married DW had a couple of XBFs who called a few times to check on her and see if she was single and interested in resurrecting a relationship with them. One of those ended up knocking up DW's younger cousin when DW refused his offer on one of those calls. DW's GM commented several times over the years that he targeted the younger cousin as a way to try to stay relevant in DW's life. 

Pretty sad for that pathetic POS and for the cousin.