Okay:
1) Just because one sibling has a play date, it does NOT mean... that the other sibling has to have one too.
2) If it is another Host asking a child over for a play-date, for one child, then that is their choice. And MAYBE it is the hosting child, that only wants... one child over. Versus plenty of kids. So the Mom could just be, asking your one child over (versus both), because her daughter only wants one friend over. The one that is her same grade/age.
3) It is not being "mean".... to only invite one child over. Unless, they are being mean on purpose and being discriminatory. To that other sibling.
4) I have tons of playdates at our house for my kids. AND when one of my kids has a play date, the other one does not or sometimes they have their friends over too. And also, my daughter has friends, who happens have little brothers too, that are the same age as my son. So SOMETIMES, my daughter's friend will come over, WITH their sibling to play with my son. Too. At the same time. Or sometimes, NOT. It is up to me. Because I am the "Host" of the playdate. And sometimes, I just do NOT want... TONS of kids in my home. And other times, I or my child, only wants THEIR own time, with their friend. It not always having to be.... a GROUP of siblings too coming over. Just by default. Each of my kids.... CAN have their OWN TIME, and their own playdate... with their friend or friends. It does NOT have to be... .ALWAYS the other sibling having to have friends over too at the same time. Just because the other sibling has a playdate.
And my kids have NO problem with that. My kids will even tell me "Mommy I want my own time today with my friends....." and its fine.
My kids KNOW and understand... that they are not siamese twins and they are not joined at the hip and they have their own friends and their own, playdates, too.
With families with more than one child... it is unfair.... to assume that just because one child has a play-date... that the other sibling(s) HAS to go on that playdate too, with their sibling. Just because that other family has other kids too. And it is unfair, to assume that that other family is being "mean" just because they do not invite ALL of a family's kids, to a playdate.
There are times, that my daughter has a friend over. And that friend has a brother same age as my son. But I tell the other Mom "Sally wants to invite Erica over, but today is just the girls..... not the little brothers. Is that fine?" And the other Mom COMPLETELY understands and it is just the way it is. Then, there are other times, that we invite my son's friends over... AND I will ALSO say "your other kids can come over too..." and then I have like 6 kids over at my house. Or I will say, "Today is just the boys over.... not the sisters...." And it is fine.
But that is at MY discretion. And my kids choice.
I always speak to them... .about social matters.
My kids are 6 and 10 and they completely understand.