How does she treat you in person? Many people MIL age may not know the nuances of text.

If anyone actually took the time to read the replies you would see that many pages ok the OP actually said she understood and realized based on the responses that MIL wasn't slighting her . Not too sure why everyone is still piling on OP but basically saying the same thing.

There is no issue here! I was waiting for there to be some kind of issue, but there just isn't one. MIL probably doesn't even think of group texts (maybe she doesn't even know how to do one). Calm down. She is not ignoring you - she is just communicating with her son. My parents do this with me all the time.