Only invite the twin sister?

Diplomacy usually involves compromise. Why doesn't your daughter want to invite her twin sister? If she's not nice to her, you have to respect your daughter's decision. However, if it is due to the maximum number of guests, I recommend a diplomatic compromise: convince your daughter to invite both twins and allow her to add another guest of her choice to the birthday party in exchange for her concession.

You are faced with a classic decision-making dilemma: whatever you do, someone will be unhappy. There is no diplomatic solution here. It's up to you to decide whether your daughter's wishes are more important to you or the feelings of the twin mum. Your daughter, on the other hand, has to reckon with the fact that either both twin sisters won't come or one of them will be angry with her - and she has to ask herself whether she wants to put up with that.
No, there is no diplomatic solution. Let me illustrate this with a joke: A woman comes to the rabbi. «I have a rooster and a hen,» she says. «I have to slaughter one of them. If I slaughter the rooster, the hen will be offended, if I slaughter the hen, the rooster will be offended.» The rabbi thinks for a long time, then says: «Slaughter the rooster.» The woman is indignant: «But then the hen will be offended!» The rabbi thinks again for a long time, then decides: «So slaughter the hen.» «But then the rooster will be offended.» «Well,» replies the rabbi, «then let him offend himself.»

