@Onlythedoglikesme
Thank you for your post. A few years ago, my adoption agency (in the US) sent letters to my BM. We never got a response to those - I have done more digging myself and could reach out to the family again but I have hesitated. I am exactly your age.
I am constantly being told it's 'my right' to contact them and to get information (even, now and unexpectedly, from my adoptive parents) but I am really aware that it could be like throwing a hand grenade into someone's (multiple people's) life when I'm not sure I would even want something long term.
I can't bear that thought, equally I can't bear the thought of my mum, having received those letters, up at night worrying about how I could turn up any time and destroy the life she has built.
I also found out that all of my BM's daughters share the same name in some form. My name is 'X', my first half-sister has my name as a middle name, and my youngest half-sister has my name a first name! I cannot even imagine a reunion like that - "Hi, here's the Real X." Given how many years ago it was it COULD be a co-incidence, maybe not. THey didn't give much information out to anyone (and when they did much of it was lies or irrelevant.)
My adoptive mother also made a huge deal about my name growing up, that she'd ALWAYS wanted to name me X etc etc. Part of the hagiography, so to speak. Once I found out about my birth sisters, I began questioning that story, and felt like I could conceivably have been 'stitched up' by both sets of parents with some fake story.
If it helps, finding out about my birth family was more about gaining control over my personal history. I didn't like the fact that people knew more about me than I did. I wanted to feel in control of the narrative.
I suspect your birth sister is reaching out now because she feels time may be running out to meet your mum. Sorry to put it like that but it's probably hit her that if she leaves it much later the chance may be gone. That has occurred to me too.
I would love nothing more than to meet my birth siblings. I have a half brother too. If it helps at all- I am sure she "comes in peace".
I don't blame you at all for being unsettled about this and your aren't responsible for her feelings.
I would also second counselling because I think in your shoes it's worthwhile focusing on your relationship with your mum. As others have said - it's very, very likely that this will settle into (at best) contact a couple of times a year after this initial period. You'll still be left with your mum and your relationship with her.