Is it too late to invite my boss?

I would

I don't think it's to late. You are the best judge of whom you want in attendance. Congratulations

Have a great wedding day

If you are going to do this (personally I wouldn't since you are less than 3 weeks out)

Acknowledge that its really short notice, and that you would really like them there

Your wedding is so close. And this point, your boss knows he/she isn't invited. Isn't your RSVP date really soon?? It was very nice of them to get you a shower gift, but don't feel obligated to invite them because of it. Are you close enough with this person that it wouldn't feel awkward to personally say to them, "when I first sent out my invites, I didn't realize how close we would get so soon. But I would love to have you there if you're available?" IMO, IF you decide to this, I think it should be a verbal invitation like that with hand delivered invite. It needs to be made clear that this isn't a b-list situation. It just sucks you didn't think of this a few weeks ago. It's just so close now!

Yeah.. I mean I don't want to come off as rude inviting them last min like this, but I also feel so bad that I didn't invite them earlier. I wish the timing had worked out a bit better.

I think in this situation, it would be ok. I understand why you didn't invite them when the invites went out - you didn't know them that well yet. This isn't a B-list. I say go for it!

ETA- I agree with @Beutivant - I would hand deliver the invite and tell them that you feel like you have become close lately, and would be honored if they could make it.

^ that's pretty much exactly how I was planning to invite them if I decide to.

My former boss & manager gave me a wedding gift a few months ago, as a "going-away" gift since I had quit & was moving away. While I was very surprised/touched by that, I had never intended on inviting them to my wedding. I know your situation is a little different, yours being a current employer, but at this point I think it's too late to invite her. I know that my current coworkers will throw me an office shower, since that's something we do for everyone who gets married, but none of them are expecting a wedding invite.

I would imagine the relationship dynamic is different since she is a nanny, working in their home. Not in an office.

Yes. I know that they want me long term. The babies (twins) are 6 months today and they will probably want to keep me until they are in school. I felt upset about not inviting them even before they gave me the gift. We have just gotten really close and will be a big part of each other's lives for a long time to come, so I just wouldn't want them to miss this, just because our relationship just started.

Very true, I'm sure it's different than a typical office dynamic. If you're going to invite her, I would do what @Beutivant said.

I think it's perfect!

I had a new coworker start at my office one month before my wedding. All of my other coworkers were invited (there were only 8 of us), and when she and I clicked right away, I realized I wanted to invite her. Once I had enough declines that I had some wiggle room, I decided to invite her. I handed her an invitation and said something along the lines of "I know this is last minute, but I've really enjoyed getting to know you recently and wanted to invite you to my wedding."