This is LONG but I'm going to tell you my experience and it might help in your situation so just take it for what it is:
Your sister is going to eventually know that you are pregnant, especially since you are close. I think best way to tell her is that you understand this is a very hard time with her for her possible endometriosis and you wouldn't want to tell her about your pregnancy except that you'd rather her hear it directly from you now than find out about it from someone else or only after seeing your bump, etc. Just explain that at some stage, she's going to know, and you wanted to just talk with her in private to tell her and let her know it is okay if she is a bit upset.
I actually had possible endometriosis for 10 years. Absolutely horrible, often ended up in the hospital on morphine. I originally started on a nutritional program to ease the pain but also wanted to safeguard any future chance I had of conception and nothing medically done had helped me. I did three separate nutritional programs, the third under the direct care of a trained naturopath who had to have THE most amazing supplements on the planet. I had damage to my liver from all the pain medications I used to take when I first came to her. After a month I no longer had an inflamed liver and my levels checked out as normal, hormones had balanced (as liver regulates hormones). I stopped having erratic, highly painful periods (and when I say erratic I mean that when I was 19 I had my period every single day for four entire months straight and it was black and looked like coffee grounds so I'm not talking about some strangeness with a mostly normal period, I'm talking about I had a serious issue going on in my body) There were many points in my past where I could have and probably should have gotten pregnant but didn't. I started to doubt whether or not I could. When I went through this nutritional program, when I was at the tail end of it, I successfully got pregnant. I wasn't even trying to. My step-sister also has been diagnosed with endometriosis and was so happy for me to hear that I am pregnant but admitted she is jealous because she has been told her chance of having a baby is "slim" I was like, "That's BS. Don't listen to the doctors." I directed her to the website where I first started getting my supplements and teas (https://natural-fertility-info.com/ not sure if I'm allowed to plug a site on these boards but I swear I do not work for this site in any way shape or form lol, just trying to help) and I told her to start on raspberry leaf tea and then do the liver cleanse kit and there's also an endo kit. The nutritional program I ended up doing this last time wasn't from this site but that website is where I got started, and I would suggest it to your sister if I were you! It could really help, and then I suggest she gets in touch with a good naturopath. They are NOT quacks. Believe me, after 10 years of dealing with MDs that had no idea how to fix me except pain killers and possibly operating on me and then I go a totally nutritional and naturopathic route and have my severely painful periods disappear and in their place have light, normal, regular periods, I have so much faith in nutrition and naturopathic methods because THEY were what produced results for me. No one can convince me otherwise, and honestly, it's not expensive to go see a naturopath or to get some of those teas and kits off that website and it's worth a shot. It's far better than living in despair and hopelessness that you will never be able to conceive.
So, when I told my step-sister, I actually had no idea she'd been diagnosed with endometriosis, but as soon as she told me that, I told her to do exactly what I did, take the supplements I took, do a liver cleanse, take high quality vitamins, cut out processed foods and eat really nutrient-dense foods, etc. I gave her a whole lecture lol. But it made her feel better to see that I had something similar and I overcame it and got pregnant so quickly after doing it. It is entirely possible that she could have enough improvement from nutritional therapy that she could conceive. So when you tell your sister, tell her you are so sorry to have to tell her this but you'd rather tell her now yourself than her find out later, and also that you have some suggestions you learned from a crazy girl who writes way too much on the pregnancy forums about her own endometriosis and what she did to overcome it
Oh and just for the record I never had endo confirmed because I never had health insurance to get a biopsy. I just kept ending up in the hospital. But there IS light at the end of the tunnel.
I know your sister might be a bit disappointed (or more than a bit) but I do think she'd be more upset to learn it later on or even from someone else, so it IS a conversation I think you should have, gently and compassionately, and I would definitely suggest these things to her to give her a possible route to pregnancy. Like I say, I wasn't even trying. So I really do believe in the power of nutritional therapy to restore your body's own ability to regulate itself, plus I think endometriosis as a disease is far more common than it used to be and that probably has something to do with the increase of processed foods in the society. I won't give you my whole shpeal but just suffice it to say it worked for me and it's worth a shot mentioning to your sister, if only to show her that you care and you're trying to help her and you're showing her compassion.
