Like @MATTERHORN said, it doesn't matter at the gate (unless it's a SoCal ticket which they do ask for ID). However, if you do want to change it just for peace of mind, then you can use the chat on the Disneyland app and they'll change it for you. It was very easy!
I had a ticket accidentally go in my name but it was for my mom. I only changed it because she gets DAS and I wanted to make sure it was all correct before she did the DAS setup.
Does your SIL already have an MDE account? I’m not sure the potential complications at DL. At WDW we did similar to my niece’s name/ticket and it caused some issues in the ways plans showed up and who could access them. A quick call to IT was able to resolve our duplicate name issue.