I think it's common sense to have children of a similar age for playdates without including older or younger siblings. My grandchildren are 6 and 9. Each one has play dates with children their own age. Yes, the excluded one complains but then when, as it does happen sometimes, when the other sibling is around for their play date, they complain about that too.
At ages 10 and 7 as well as 8 and 5 they have different social skills even if they have similar interests. They also need to learn who they are separate from their sib. They have to play with their brother most of the time. It's their turn on a play date to play without having to include their sib.
My daughter does have play dates with sibs with her 2 children but the children as similar in age. The children staying overnight or playing on Saturday afternoon are 10 and 7. My grandchildren are 9 and 6. The children are at or near the same level of development. The ages 10 and 7 compared to 8 and 5 may not be so compatible. A 5 year old is definitely less likely to have the social skills of 7, 8, and 10 year olds. There is a big difference between ages 5 and 10. What works depends on the children and the parent's ability to manage the time..
If the four are not so compatible it is much more difficult to keep things running smoothly. This may be another reason to split up the sibs. I do include another child when I have one of my grandchildren with me but I will not have 4 children at once. I tried that and was worn out each and every time. Not all adults have the same capability to handle multiple children. This could be another reason.
My daughter at times tries to arrange a play date for one of her children at someone elses house at the same time she has a play date for the other child at her house. The two children in one family of the above example is one that works.
I think it's important to be flexible to allow for individual differences and abilities.