I don't see any problem with, occasionally, making plans with just one's parents, or even just one's mom, dad, brother....
If the three of these are so dysfunctional that that they just can only come as a package, then that is the issue.
I don't think that it would go over to have a blanket exclusion of BIL.... never want to even see him. Family occasions, such as family getting together for Christmas, should be considered as 'family occasions'.
I would be looking for occasions, such as, as a gift to my parents, taking them out for a special holiday brunch... Their specific birthdays.... etc...
Things that would qualify as special gifts/actions for the parents might be good opportunities.
Also, if BIL has any kind of a life, whatsoever... look for occasions when you know he will not be available.
How do the parents feel about the issue.
That really dictates how to handle it.
If they are normal and reasonable enough to want to spend time with both of their children, then okay.
If they have some co-dependent dysfunctional thing going on with BIL, and would never do anything at all without him... yes, that would hurt... but that would be their decision.