Brother in law's wife has decided to be bridesmaid

[titzillaa]

long time lurker, so I know what the consensus is on in law wedding threads, but I have an inlaw wedding dilemma!

so me and DP have been together on/off for six years and engaged for two. we've finally set a date for the wedding and started planning, I've decided on eight bridesmaids (my 2 x Dsis, my cousin, four friends and DP's sister who I'm very close to). however, DP's sister texted me earlier announcing that she's pregnant (due january) and so she's not going to be able to be a bridesmaid, as she doesn't want to have to worry about fitting into her dress and breastfeeding a newborn on the day.
fine - I have absolutely no problems with that and was very happy for her, and my plan was to just have seven bridesmaids which is plenty. however, DP just got home from work and said he'd had a text from his brother's wife (she's very very close to his side of the family) saying she can happily 'take over' from SIL?
firstly I don't see why she needs to 'take over' as it's not as if there were any specific duties that SIL had. secondly, brother's wife has already complained about the date of our wedding (it's the day after she gets back from a music festival), the venue (it's the one that her and BIL were looking at for their wedding reception last year, but it was booked and they ended up finding one they liked more) and said the colour of the dresses was ugly (they're khaki). she's not said any of this to me but instead to MIL/SIL/BIL who have relayed it back to us.
however if I tell her I don't want her to be a bridesmaid she'll cause a massive fuss (she has very few close female friends so I was a bridesmaid at hers last year - more out of filling space than intimacy) and DP's family will take her side, I know it.
AIBU to be pissed off?

From your OP it sounded like she was just offering to fill a gap, like you did, not asking to be a bridesmaid?

I had a very discreet wedding - but I love more flamboyent affairs! I hope they will all be in matching khaki tiaras.

www.lexico.com/en/definition/sister-in-law

In German there's actually a special word, Schwippschwägerin, to describe a SIL who's your BIL's wife. English is lacking in that department!

For the sake of peace in the future I'd include her.
You were a bridesmaid to her.

OP - it’s no one’s business how many bridesmaids you choose to have. However, I do think it’s relevant to the situation here.

One or two bridesmaids (let’s say a sister and your closest friend) would have said ‘intimate wedding’; it would have been understandable that you didn’t choose her even though she chose you. Eight bridesmaids says ‘Even though I’m having more than double the number of bridesmaids most people have, you’re still not important enough to make the cut’. Now you’re planning on saying that you’d rather have a spare slot and an unused dress than involve her. It’s a pretty clear message.

Also, be wary of basing your opinion on second-hand versions of what she’s said. Was it ‘They’ve booked it for the day after Glastonbury?! They KNOW I’m going; why are they being so selfish?!’ Or was it ‘Oh shit, that’s the day after I get back from Glastonbury; that’s going to be a bit tight’. One is making your wedding all about her; the other is a natural reaction to something being a bit tricky logistically speaking.