She sounds like she is .reaching out to your husband through you. And that might be understandable -- maybe she figures that the occasion of a shower is a reason to open this door. There's nothing wrong with that; she might be a terrific person who is at a point in her life where she's realizing she missed out on knowing her half-brother, and who wants to start slowly reaching out but also is afraid of rejection if she flat-out phones him and says, "Hi! It's me!" You cannot know her motivation here but why not assume the best, and give her the benefit of the doubt?
The key thing here, which you don't mention at all, is how your husband feels about this invitation and, more importantly, about her being in touch at all. He wasn't invited possibly because it's more traditional to ask only women to some showers, or maybe she feels you'll relate better to her since you're both women--who knows? Maybe .she is worried that he'll be baffled if she approaches him after this long so she'll approach you first instead. Again, nothing really wrong with that; it would be nice if she felt she could just contact him again and say "I'd like to be in touch with you" directly to him, but she might be .worried that he'll reject that out of hand.
I would send her a very nice card from both you and your husband, absolutely, and a gift card for her registry if you want. If husband wants to be in at least minimal touch with her, see if he will write a brief note in the card. People often blank out when this kind of thing comes up, but it's enough if he just says something like, "It was a nice surprise for us to hear from you and we hope things go well with the baby. My e-mail address is XYZ and I'd be glad to hear from you again." If he doesn't want to be in touch at all, you send the card and gift card yourself.
Have you told your husband about this and talked with him about opening this door? .This is going to dredge up some hurt for him, but I hope he doesn't blame her personally for that hurt; it was his dad who was responsible, not a half-sister who apparently is quite a bit younger. You note that she is in her 30s and hasn't spoken to him "in 25 or more years" -- so you do realize that she was still a child when for whatever reason, she and/or her family stopped speaking to him, right? And if she hasn't spoken to you "in the 20 years" of your marriage she was just a child or teen when you got married. I hope you and husband see that the math here means she, herself, likely didn't ever really know this older half-brother very well; he got married (and moved away?) when she was still young enough to do what her parents told her, and also young enough that she probably didn't know or understand his hurt.
I guess I'd just say, give her the benefit of the doubt, and don't let the decades of silence prevent your husband from getting back in touch if he wants. She can only have been a kid back when he was going through the pain his father inflicted. She might never have been told much about your DH, or told things that aren't true, and maybe now she's realizing he's part of her family too.