Whoa there. You need to take a chill pill. (Am I the only one who still says that phrase?)
You're making way too much of a big deal about these things. Like a super over-the-top big deal.
I could actually be your SIL, sort of. DH and I also had fertility help to conceive and we're also due in early June. It's our 2nd though. We had help with our first and took us a long time to conceive.
I think maybe my sisters and SILs know that we had to have help to conceive, but my mom and MIL don't know. I just never felt like telling them or that they needed to know. It's almost like I don't want to trigger the over-protective, over-inquisitive nature of mothers maybe. My mom is a bit of a blabber-mouth, so I know everyone she ever meets would then know that I took fertility drugs. Also, my mom is a worrier, so when we started the process I would worry that she would voice the scary possibility of us never having children or not being able to have children, etc. It was better for her that she didn't know. MIL is just too nosy. She's ask a billiion questions, and ultimately she'd hone in on the fact I took Metformin, which is a diabetes drug, and that maybe I should eat more vegetables instead of bread. So that's why I didn't tell her. And trust me...this doesn't just "come up" in conversation. It's a private issue and you should never bring it up. If your SIL brings it up in front of MIL, it's her issue.
Now about the registry. For my first, I didn't even start a registry until the 3rd trimester. And even then, for a few weeks it was a work in progress that I just used to keep track of the items I had researched and was pretty sure I would get. I didn't tell people about it because I was not ready for, and did not want, people to buy things off of it. I have a registry now for the same reason. It's more of a reminder shopping list for things we may want/need this time around. I'm guessing your SIL just recently started it, but it's more of a work in progress than a definite must-buy list.
So simmer down. You have a good 10 weeks before you need to buy anything for her. If you want to buy her something special, I'd ask her if she's having a shower and when that is scheduled. You won't be able to attend becaus you're on the East Coast, but if she's having a shower in April or May, you can plan to buy and send your gift about 3 weeks before the shower so you have top pick of the final items she has registered for.