user I am sorry you are upset.
I agree with the gist of meercat23's comments, except to say that my adopted son is very much 'my own child' and parents of adopted children usually feel this way, even if our children do still have another parent or parents out there somewhere who is also their parent.
BlueberrySky it's not usually a first choice but when one gets to that point and it is the only choice, as it was for me with our son - to have a second child, it really doesn't matter how you get to that place. Or at least it did not for me. Ironically, I knew I wanted to adopt before we got to the point where we actually 'needed' to adopt if we were to have a second child.
User you said, your relative has a condition that may cause fertility and that may mean she cannot have children biologically and you said she '... is understandably upset'. So I think you must just accept she is struggling with that. Whether she will adopt or not will be her choice. I think instead of being pissed off why not just keep on thinking you are 'pretty bloody lucky' to have your kids in your life and don't think anyone should feel sad about ending up like you.
Your situation is your own, your young relative may or may not end up with a family through adoption. I think your other relative is the one who maybe should take a step back and not try and adapt your story for themselves!
You said "I'm genuinely really sad to think that I am perceived by other people (particularly family) as someone who's life is somehow less because I can't reproduce kids." But do you know that that is how people feel or is it just the one suffering from a medical condition that could cause fertility issues?
I've got several family members who did not have kids, they may be very happy about this, we don't discuss it, but I feel kind of sad they did not have kids as that was so important to me, but I recognize it may well not be the case for them! I'd never say so to them. They have filled their lives with overseas travel or being fabulous aunties or whatever.
As an adopter I know it is really just not so simple. My adopted ds is really quite easy to parent compared to my birth dd. Birth dd is a handful and I am sure some people feel sorry for that! DS can be a bit of a handful in a small way too! But really my kids are my life, my world revolves around them! Whatever anyone else thinks about my 'quite difficult' kids, like you user I would not swap them for anyone!
I think the only way to get this message across is to talk within your family about this, but do remember your relative is thinking about herself, and how she feels, and to find out at a young age you may not be able to have children biologically could be quite devastating. The fact you were not devastated by it is to your credit but does not take away your relative's right to feel like this.
Please also be careful if all this is coming through a third party (other relative), she may have got things a bit wrong, who knows!
Re "... the denial that the fertility issues are partly me, makes me feel like it's something to be ashamed of and that it's all about blame"
I think you have to decide whether you want to share your information about your own condition with your young relative, and whether or not to include your other relative in this or not. Of course fertility issues are nothing to be ashamed of, but you just need to decide how much you want to share.
I have had fertility issues and I don't think it is anything to be ashamed of at all, I've generally been very open about it and I know I will need to talk to my dd (I did manage to have one birth child with fertility treatment so she shares my biology) because the condition that runs in my family affected me, my mum, and my cousin. It won't affect my adopted child because he is a adopted - and most of all because he is a boy!!
I understand where you are coming from because I would not want anyone feeling sorry for me because our son is adopted. He is amazing, we love him, we could not be happier. But I think your relative is feeling sorry for herself, she is worried she may be infertile and is taking the fact you adopted to mean that you could not have children biologically because of your condition, it's really not about your kids or about you (IMHO).
Good luck.