DH and his ex have a son, they have been split for about 6yrs. Ex is also re-married. DH has son every weekend and half of school hols and normally does all pick ups and drop offs. Timings/days have been flexible though depending on what events are happening in either family (weddings/birthdays etc) so DSS may spend more/less/different time(s) with us if something special is happening and sometimes his mum will come and pick him up if she's changed things making it difficult for us to drop him off at a different time. This started off ok but we've run into problems over the last few years.
Basically his mum keeps "invading" and damaging our special times, and it's really bothering me. So far there's been:
DH's sister's wedding - ex was actually invited but declined. We had DSS with us, she picked him up, he was knackered but she decided to stay for 2 hours carrying round a sobbing DSS who just wanted to go home.
My best friend's wedding - she picked DSS up, after the last time we did the handover outside but she decided to burst into the evening reception sobbing hysterically because one of her ex-friends was also there.
Our wedding - she refused to set a time to pick up DSS, instead insisting that we kept in mobile contact all day and rang her to collect him. Then proceeded to tell DH off for an hour outside while everyone was inside wondering where he was.
Birth of my DD - came to hospital with DSS but rather than leaving him with dh's family and waiting outside, decided to come into my cubicle with DSS so my family couldn't get in as we weren't allowed any more people there.
My 30th - by now we were getting wise to her creating scenes when she picked DSS up so we asked if he could go home later instead of being picked up by exW - she said no. Turned up and sat right in the middle of my family and sat there ridiculing all of my choices, how I dressed DD, what I fed her etc. Told me I was a control freak because I fed DD some veg 
We have got DD's 1st birthday coming up and are having a small family gathering. DSS' drop off time is in the morning meaning he'd miss the party so we've asked if we can drop him off later and told ex when it finishes. She's said she wants him back by a time which falls right in the middle of the party meaning my DH would miss most of it. She has offered to come and pick him up but I really don't want her to do this given her past form - but I also don't want DSS to miss out because of his mum's awful behaviour.
These are the only pick-ups she's done since her and dh split but they always seem to be dramatic - she seems to put us into a position where we have to let her pick him up by not allowing us to change our timings with DSS, this forcing dh to either leave the event halfway through or stick to normal timings but rely on ex to pick up.
She still sees dh's family and their mutual friends so it's not like when she comes to these events it's unpleasant/awkward in that sense. I can't think of any reason for it apart from getting attention or just causing trouble. Worst thing is she comes across as a nice person in general and always apologises which dh always falls for (massive guilt issues) so he always lets her back into a situation where she can (and will) do it again. Apart from excluding DSS as well (which I don't want to do), I don't know how to keep her away - what should I do?