I’m not sure why you would be offended or find it weird if your MIL wanted to spend time alone with her biological daughters by herself without including the DILs. She didn’t even invite her son (ie. your husband). And perhaps she lied b/c she wanted it to be just them and didn’t want to take the risk of you/jethani saying “yes” to coming. Sounds like her only mistake was mentioning this to her son b/c she obviously didn’t expect him to bring it up.