OP, as someone notes above: You gave a "maybe" but that only leaves BIL thinking you might do it. Please change it to a clear "no." Of course going to NYC on short notice, sandwiched between other trips with a kid in tow, staying in an NYC hotel ($$$$) -- it's a no-go. Are you concerned that BIL (or SIL ) will be upset? Eh, this is really on BIL doing things last-second. The fact you got added in the middle of a longer text exchange between BIL and someone you don't even know is an indicator it was a last-minute thing arranged by folks who dont' have kids and/or other plans already made.

Give BIL a very firm but cheery nope, can't make it, but you have a good time and thanks for thinking of us. Then if you are going to see SIL at the Halloween visit -- not clear to me, does she travel to meet you for that one? -- take special treats as well as gifts to make the birthday special. Big basket of pampering extras she would like, membership to something she loves like a museum, gift certificate to do something she wants to do but normally wouldn't splash out for, whatever. Better way to spend your money and time, than to spend them on a surprise trip that might or might not be as welcome to her as BIL thinks it will be. He means well but doesn't get that popping into NYC when you have other trips so close to that is not feasible.

Anonymous wrote:Just say no to this second trip. It is dumb. BIL sounds well intentioned but clueless.

This. You already said you might but make it. Change that to a no.

I have never traveled for a siblings birthday or for a BIL or SIL. It’s too hard and how impossible with kids. If I happen to be there that weekend I’m happy to celebrate with them. Say Happy Birthday to her over Halloween weekend and offer to treat for dinner. If she has plans do not feel guilty.

Send one adult--you or DH--not the whole family.

Agree to say no. BIL sounds very sweet but to be honest, I think SIL might prefer a dinner in NYC out with just her college girlfriends instead of having her brothers crash the party unexpectedly.

Just say no to this second trip. It is dumb. BIL sounds well intentioned but clueless.

We live 4 hours away from my husband's hometown (where his parents and brother's family live), and 3 hours from NYC (where his sister lives.) Every year we travel to his hometown on Halloween so our 3 yo daughter can spend Halloween with her cousins and to celebrate his sister's birthday (also on Halloween). This year is her 40th and she also just went through a breakup and so BIL texted us a week and a half ago about setting up a last-minute surprise for SIL to make her 40th extra-special, especially after her breakup. I texted it was a great idea and maybe we could do a surprise dinner out (we usually just have a low-key dinner at her parent's house). He said SIL's college friends were going out for dinner the following weekend (Nov 5/6) and maybe we could surprise her there, maybe in NYC, but he wasn't sure of any details. We left the discussion there and didn't make any further plans.

Fast forward to yesterday when we got added in the middle of a text conversation between the BIL and SIL's college friend, discussing hotel options in NYC and asking where we wanted to stay. DH's parents were also included and had no idea what was going on. I texted BIL and his parents separately saying something like "I'm not totally sure we can make the NYC weekend, but maybe we can plan a dinner out the weekend we're in town for Halloween." Then his mom also texted saying she didn't feel up for a trip to NYC (she had major surgery a few weeks ago and is still recovering.) BIL texted back that we should try to make the effort since it's her 40th, and her breakup, etc.

Our issue is that we are already traveling for Halloween, then we also have travel plans with our daughter the weekend after this NYC party, and then the following week we're traveling to my parent's for Thanksgiving (also far away.) I just feel like it's too much for us to be traveling every weekend in Nov and especially since we're already in town for SIL's birthday, we can just celebrate with her then and not make another extra trip. But I know BIL is just being a sweet brother and trying to set up a big surprise party to help his sister feel better. The only option I can think of is to cancel our Halloween trip and just do the NYC trip instead, but then SIL might feel slighted we weren't in town for her actual birthday.... plus then we don't get to see DH's parents or do the trick or treating with cousins. The other options is just to have DH go to the surprise party in NYC. Or put our foot down about planning something with just family the weekend we're actually there. TBH I'm not sure, given SIL's personality, she would actually want her siblings and their kids surprising her by coming to a dinner with her college friends anyway... if it were me, I'd want to have that time just with my old friends.