Whilst I don’t think you are being heartless at all chinesecrested (you have to do what you have to do) I couldn’t do it. I always have my 3 all together since they were all just weeks old (unless there’s a reason why one can’t come obviously). They are 12,10 and 8 and they still fight occasionally. They have gone through the toy grabbing and spiteful stages but it would break my heart to take one and say no to the others as they would wonder what they had done for grandma not to want them.
Could the way forward not be making rules for play and you each giving the children some special time alone (in another room maybe). From memory there are lots of things a 2.5 and a 5.5 year old can do together and I wouldn’t allow the older child to think that you belong to him alone and his sister can’t be part of this “special relationship” . She will grow up feeling second best. Good luck whatever you decide.
Is there such a thing as delicious ready meals?