| Wow OP, I am so embarrassed for you. |
| I'll be the lone minority here. I think it's odd there's a joint party for someone who isn't really part of the family and your sil's response is rude af. |
You can "think it's odd" all you want. You can NOT ask for two parties to be hosted by someone else because you think it's odd. |
| You should have just told her the truth. You are cheap and not a nice person so you can’t attend. |
Maybe it is odd. But there's no way that it was appropriate for OP to ask her SIL to host TWO parties instead of a joint party so that she doesn't have to sing happy birthday to a little girl she doesn't know. |