- I think it was unreasonable of your friend to put you on the spot by putting that on you at this stage. Unless you were both good friends with the other woman.
If I had a very good reason to want to include the other one, the most I'd do is mention that I think they'd like to come but that it would change the dynamic. That would make it easier for you to say no by just agreeing with that.
If I was feeling like you are I would have said I wasn't keen on changing the dynamic this time but, as part of that conversation, I would naturally also have asked my friend why she wanted to bring the other person.
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Your feelings of unhappiness at having been put on the spot are NBU.
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But assuming the worst about why it was asked is unreasonable (to yourself). If you cancel you will never know and will continue to feel awkward with that friend.
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I agree with those saying some posts are harshly worded (given that you posted because the request has knocked your confidence in the friendship a bit}, the ones suggesting you could be a vampire friend.
It could just as easily be nothing like that at all. So please don't take that all badly to heart.
If you were already wondering about that, you could ask your friend as part of conversation at the brunch. But only if you feel able to accept hearing it if it's true, and only if you'd feel able to recognise it as her just being honest about her own limits/needs rather than trying to hurt you.
I mean ask her calmly in an "I just feel I'd better check not piling too much on you" way, not accusing her of judging you. But only if you think there is a possibility of it. It sounds from your posts as though confiding/venting has been a 2 way street in the friendship so there's maybe no need to ask unless you pick up any vibe that she's feeling differently at the brunch this time.
TLDR:
Your friend was unreasonable to put you on the spot like that. But since you don't know why she asked, it's best to not even try to second guess it.
If I were you I would tell myself to assume the best and go to the brunch but, if anything on the day made me think that maybe it's true that she wasn't up for such heavy discussions, then I'd ask her about it, in a "wanting to check it's not overloading you" way.
(But I would have already asked her why she wanted to add the 3rd person anyway, when she asked. Not by messaging though! I would have tried to find a time that was OK to briefly phone her so I could ask as part of replying to her, with no risk of it being read in the wrong tone).