(Nick Galifianakis for The Washington Post)
Advice columnist

Adapted from a recent online
discussion.

Dear Carolyn: I'm expecting our first child. Since we announced my pregnancy, my husband's twin sister has treated all baby decisions as joint decisions between her, my husband and me — everything including the brand of prenatal vitamins I use, circumcision, baby names and car seats.

For the 20-week appointment, we invited our mothers to see the ultrasound. Sister came along as well. When I was making my next appointment, she wrote it down as if she planned to attend.

At first, I just said neutral things such as, "Thanks for the tip/article, we will consider it." But now she contacts me daily about the baby and made it clear she expects to be in the delivery room. Her intentions are good and we don't want to hurt her feelings, but my husband and I both feel a little smothered and I definitely don't want her in the delivery room. How can we address this?

— Baby Makes 4

Baby Makes 4: Oh my, no.

There’s never a bad time to set a good boundary, but sooner is always better. I realize shouldas are evil, but many see babies as invitations to get up in your business as far as you’ll let them, so rethinking your choices will be useful for this future. Apply this shoulda hereafter: Nip intrusions early.

And acknowledge the difference between overeager bystanders and full-on invaders. I am a huge fan of your neutral deflections, but when the lines are crossed this far, firmer limits are necessary.

Coming along on the appointment uninvited, for example — that was a time for your husband (the better messenger here) to talk to his sister. Each vitamin/car seats/circumcision discussion is minor, but in aggregate, they represent both a serious problem and missed opportunities to catch it early. “Sis, you’ll be such a great auntie. But I’m not comfortable opening up our decisions to family review like this.”

It will be harder now that her perceived investment has had time to grow. But that also means you don’t want to stall anymore.

Because she’s giving you so many opportunities, you might accomplish more with Small Talk(s) vs. a Big One. Meaning, one of you responds in the moment to one of the big boundary crossings — “Thanks, but this decision is between us” — then you wait to see if your message gets: (a) ignored; (b) received, but applied in this one instance; (c) received and applied widely.

If (c), then, yay. You’re done. If you get (a) or (b), then set a firmer boundary at the next opportunity and see where it goes. If she gets upset, then it’s time for the full-clarification, we-love-you-as-a-sib-and-auntie Big Talk: No third parents, please.

Re: Twin: My husband is very close to his sister, who's a veteran mom, so he spent several months assuming I appreciated all his sister's input — when I really wanted to experience being a first-time mom, including the clueless decisions and mistakes, without being constantly reminded that someone else knew better than I did. My husband was shocked to hear this but quickly reversed his approach once I found the words to say so. Good luck!

— Anonymous

Anonymous: Congratulations! On the baby, and on finding the words, and to your husband for being willing to pivot like that.

Write to Carolyn Hax at tellme@washpost.com. Get her column delivered to your inbox each morning at wapo.st/haxpost.