I would tell him that the last time you all made plans he flaked on you every single time. Up to and including saying he "forgot" about you. Let him know that he and his family are always welcome, you would love to see him, but you can't keep your family hanging only for him to bow out at the last minute

Tell him that you would love to see him and his family. But since he acts like the only relationship he wants with you is cursory and superficial, you are not assuming he will show up until he's on the doorstep. It's up to him, if he wants to be "Christmas card family" he needs to let you know so you can manage your expectations accordingly.

Tell him the whole beach thing has hurt your feelings. Forgetting the dinner plans and telling you he was being seated at the restaurant is beyond rude. Would he treat a friend or colleague like that? I would say definitely not. He has treated you like crap and needs to be called out on it.

I would just be brutally honest and say something like ‘ Look Fred, I understand Ashleigh doesn’t enjoy our company for whatever reason. I don’t really care and have no desire to dig into whatever is going on with her. I just don’t think trying to force something during a vacation again is a good idea. If we are all there on the same dates, maybe you and I could grab coffee or breakfast without everyone else and catch up. If that isn’t going to fly on your end, no worries I get it.

It's your brother's fault, OP, not your SIL's.
You need to tell him that he's neglecting you, and that you're really disappointed. You need to point to factual examples (SIL's ignoring you, forgetting you and your kids and going to eat with other people, etc). Tell him you're sad you're not more important in his life, because he's your only family, and you used to be close. But that you cannot be disrespected in this way again, so unless he makes more of an effort to prioritize your family, you refuse to be at his beck and call.

Not SIL fault. Say, great, text us when you are there and available to see us. Say nothing to the kids and don't plan to see them.

Why are you spending so much time chasing people that don’t like you and treat you and your kids like garbage? Is that how you want your kids to act as adults? I would say “ that’s great! So excited you’re traveling” and if they bother to call you are busy and it just won’t work, maybe next year. His family will likely be relieved.

Your brother had to choose between you and his wife, and he chose his wife. Given the length of time this has gone on, I wouldn’t expect change.

You can say great! And probably have the same experience as last time.

You can tell him that the cancellations last time were hurtful and detracted from your vacation. He will probably not be sorry or change.

I would probably let him know when you’ll be there, closer to the date you can give him your schedule and say they’re welcome to join or come over whenever, and go about your vacation with no expectations of him. Bonus if you can invite friends to come with you.

Did you not call your brother out for what happened last year?

You asked how you handle it. Say "we will be at x place at x time if you want to join us." Like, a movie. Then just go and if they show great, and if they don't, no big deal. Don't even tell your kids they might come.

Say brother, I'd love to see you, but last year when you came you forgot about us, and it didn't' really work out. I'd love to see you and the boys, but I don't want to get too excited about it as something always seems to get in the way.

How is your relationship outside of vacation times? Do you guys call each other and talk on the phone each week? Celebrate holidays together? Attend important events for each other’s children? Etc etc.

I would maybe extend one invite for a meet up during the vacation, but would focus on your overall relationship instead of putting so much pressure on a vacation meet up.

Stop blaming the SIL, this is just as much your brother's fault as it is hers. Personally, I would just say it won't work out this year and save yourself the headache and disappointment. Otherwise you can leave it up to him to make plans or not. Don't chase after him. Do your own thing and if he decides to include you, great.

But honestly, I'd just plan to do something else when he's there. He's made it clear that he and his family don't have much of an interest in you and yours, and I wouldn't sacrifice my own happiness hoping for something to change.

If you want a relationship with him at all, then you hope for the best and expect the worse. You have very little opportunity to see him. So isn't 1 hour of stress better than nothing at all.

Also, if you accept that it's going to be bad, then it might make you less of a target.

Beyond that, you should manage more, and if they don't show up, so be it. He wants to do dinner. Great. We're going to X at 6:30, does that work? If they aren't there at 6:30 call for status update.

And if none of that sounds worth it, then just say hopefully you all can make a get together work. You'll touch base once you're there.

You can't come across as needy.

How old are your kids? They may be avoiding them. I'd consider trying to schedule something adults only

Just say it won't work out. Have him enjoy his own beach vacation and you enjoy yours.

Sorry if this is long I am giving the shortened version: My brother is my only family. He and I who are close in age used to be very close until he got married and moved. He moved back for a few years with his wife. Any time we tried to get together she always would "get sick" and not be able to come and my brother wouldn't come. She is very possessive of my brother and now of her college aged son's. She likes male attention and is cold to me, but lavishes attention on my husband. We haven't seen them in years because she prefers to visit his single guy friends.
Last year my brother said they were coming to visit us at the beach where we have a house (they were renting a place). They invited one of his single guys friends to come for 2 days of their 3 day trip. We saw them for a quick dinner, they invited us to meet them for dinner the next night. My SIL never talks to me unless it is to correct something I say. The next day I called to check in on the time and they reported they had just gotten lunch at the place they had invited us. The next day my brother invited us to meet them for dinner and said his group was in the shower and would update me with a time. I didn't hear anything for 1.5 hours, my kids were starving so I called him. He said he forgot about us and that they were just about to be seated at the restaurant and couldn't ask to change the table size. I had offered every day for them to see us on the beach and they never came. My feelings were really hurt by the whole visit. He seemed to have no real interest in seeing us and his kids who are older than mine by 4 years completely ignored my kids who had been so excited to see their older cousins.

Flash forward a year and my brother told me they are coming again this year and want to see us. How do I handle this? I always end up feeling terrible after every interaction because of the digs thrown my way or if SIL invites someone she will completely ignore me.