1) do not bring your children. You need to be ready to leave if she will not behave.
2) maximize your ability to leave. Do not get overly comfortable. Do not order something that you'll have to wait for a check. Pay cash and be ready to bolt.
3) do not meet them on their turf. Be somewhere public that she/they are more likely to behave themselves.
4) understand that you cannot control her behavior. You can only draw a boundary. A boundary controls your own behavior in the event of circumstances.
E.g. we will not be spoken to rudely. If she persists in speaking to us rudely, we will leave.
Boundaries should be decided on ahead of time with your husband so you are on the same page.

Start the meeting with an explanation that things have recently crossed the line. That this conversation is to let them know because you value the relationship and you want to salvage what you can. You expect there to be respect and cordiality from yourselves and from them if the conversation is to continue. This means no name calling, no insults, no accusations.
Then you say that her behavior indicates an unwarranted claim on your time. The she believes herself entitled to know and control your schedule. You tell her that she is extended family - that she is not entitled to know your schedule, that you do not have to justify how you spend your time, that however you are spending your time is not in any way an insult or disrespect to her and that if she is going to spend the time and contact that she does have with you and use it to fling accusations and insults and be offended,, then she will find that time and contact severely reduced. Why would you want to spend a 3 hour round trip with a baby and a toddler only to be guilty, insulted, accused? Why would you want to read texts or answer phone calls from someone who is regularly rude?
If any friends treated you this way you would cut contact and the only reason things have lasted this long is because you have been exceptionally patient. That the patience is almost at an end and that you are going to require her to treat you with respect for your personhood, time, schedule, etc. if she wants you to continue to spend any of your time, schedule, lives with her.

If things haven't blown up yet, you can continue by explaining that due to the growth and stage of your family, they should be fully expecting that the amount of time you have previously been spending with them is excessive. Moving forward, your family making the drive is likely to change to once every 3 to 6 months, but no guarantees that it will be that often. And that if they want to see you more often, that the road works both ways and there are hotels nearby. That you would be willing to meet them at a restaurant halfway for a brunch or lunch once or twice in-between trips up there.

If at any time she starts crying, just tell her she obviously needs some time to get herself together and when she's ready the conversation can resume.

This is not an attack on her. You are not doing this to her. You are trying to save your relationship with her in spite of her behaviors toward you. You are trying to manage her expectations of what you are able to give.

If she starts insults, accusations, screaming, rudeness... leave. Tell her that you will not sit and take abuse. You can try to have the conversation a different time. You'll be in touch in about a week. Then put her calls and texts on silent. Don't block her. Let her dig her hole.