Having family over can be really stressful, look at Christmas when arguments are often rife because folks who don’t usually spend that long a time in close quarters start getting on each others nerves. Add in high expectations of happy family time and it can make spending one single whole day together a shitshow by the evening.
Your family will be tired after a long drive on Friday, they might be tired from preparing for your visit, and there will be lots of excitement about seeing each other, and expectations of a happy family reunion.
Having a break from each other on Saturday daytime will give your family time to settle down and explore the area, do something fun as your own unit, and (possibly) tire the kids out a little before reuniting back at your sisters for the evening.
Golfing also is giving your sis a bit of headspace in the middle of the visit, wide open spaces on the course to counter her home being full.
And it might be that kids, even well behaved and/or interesting ones, can be exhausting to be around, even for family, even if you had kids of your own even our own sometimes
Going and expecting to spend all day every day with them might not offer the same option of breathing space and tensions could flare.
Personally a little breaker in the middle sounds a good idea to me, it stops either family being overwhelmed and gives you all more topics to discuss so conversation will flow easily without straying into too personal, sensitive, or old areas of strife.
And if she sees her other DN’s more often then she is more familiar with how they are than yours so knows what to expect/has an established regular contact with them.
For example my DC is close in age to a cousin, I looked after cousin & his brother, result is I & my DC are closer to these cousins than my other DNs & spending time even now years later is just easier than the younger DNs - even though they’re lovely too & I love as family, it just isn’t the same.
If your DC miss auntie then accept the breaker on Saturday and see this weekend as a beginning, building up that relationship between DC & Aunt, so that in future there will be that easier rapport similar to with her other DNs (and bear in mind it might never be the ‘same’ but that’s no one’s fault necessarily)
Pause for thought before throwing the head up & canceling @Jennybeans401 in the long run everyone might benefit from slow, steady relationship building which will last rather than shoving you all together, forced activities and a big row when expectations are not met.