It depends on what you mean by not including you. Are you the type that can be in any crowd? Did you attempt to participate? If you were expecting for your sister in law to be more one on one with you then you had the wrong expectation.
In group events it’s hard for the person hosting to spend time with each guests. Unless she invited you and left you exclusively all alone it could be that you didn’t know anyone there and felt out of place. I would talk to her about how you felt it could have been a misunderstanding. When you go in a group setting expect to meet new people. Be open to new experiences. Sometimes our anxiety alone can make us feel alone in a crowd of people.
Unless you have the thought that your sister in law doesn’t like you, I wouldn’t put too much stock into it. Even if she invited you for the sake of her brother there’s still a sense of wanting to get to know you a little better. Group settings allow you to be able to open the door of communication where there is none. In your letter you didn’t state any one thing that was done that would be a red flag of her leaving you alone more than another lady there. This let’s me think it was more in how you felt.
However the flip is some in laws can be mean. Sometimes they want to see how you do in some settings but those situations are extremely rare. Most times it’s just the navigation in new crowds that catch us off guard. Not only that I don’t know of a real husband that would allow you to be ambushed. Your husband would know how his sister really felt about it even if he didn’t express it.
Now that she made the initial step I would invite her on an intimate outing such as lunch or maybe some one on one shopping to clear the air. I’m confident that you will find things to be okay between you.
Good luck getting to know your sister in law!