In-laws are shunning me after I (28F) refused to be an usherette for SiL’s (26F) wedding. How do I approach this issue?

Background: Hubby (27M) and I (28F) married in Oct 2022. I had good relationships with both of his sisters (26F and 19F) and spoke with them regularly. I asked them to be my bridesmaids and to choose the center diamond for my engagement ring. I wanted my SiLs to feel valued and included so I involved them as much as possible hoping to build sisterly relationships with them. My sister was murdered shortly after I got engaged. Originally, I planned on my sister being my MoH. My husband’s sisters knew this and understood why I wanted them to be so involved, and they were happy to help.

Oct 2022: The eldest sister, Elle, is engaged to a serial cheater. Her fiancé was invited to a different wedding on the same day as our wedding. Elle wanted to accompany him because she “doesn’t trust him going alone.” I feared Elle was unlikely to attend our wedding because of the double-booking, so, I changed the ceremony time hoping Elle might attend. No one asked me to do this, nor did I tell Elle why I changed the schedule. Ceremony goes smoothly, we say our vows, kiss, photoshoot, etc. Elle stays for photos then immediately departs for the other wedding. I was disappointed she only attended for an hour, but kept my feelings to myself.

Jan 2023: Elle’s wedding is set for October. All the bridesmaids have been chosen. I wasn’t asked to be a bridesmaid, but I was content being a guest. However, I was hurt that she chose the fiancé’s sister (after learning she covered up her brother’s cheating) instead of me. Again, I kept my feelings to myself.

March 2023: Elle asked me to be an usherette. I felt insulted that she asked me to be essentially a Walmart greeter at her wedding while excluding me from all other festivities with the bridal party. I know some folks say it’s an honor to be an usher at a wedding, but I honestly felt snubbed. I simply replied, “I’ll think about it.” Elle stopped talking to me after this interaction. So, I stopped making an effort to be sisterly with her. Now, we avoid each other at all future family functions. Elle would eventually call off the wedding after catching fiancé cheating again.

Dec 2023: The younger sister, Sofie, has a sudden change in attitude, cuts communication with me, and deletes me from social media. This behavior was unexpected since we’ve never had problems. She says I haven’t bothered getting to know her or Elle. I wasn’t particularly close with Sofie, but I felt we had a good relationship nonetheless. I suspect she thinks I’ve wronged Elle and is now parroting her sister’s feelings.

My husband graduates from medical school in May but has forbid his sisters from attending because of how they’ve treated me. This greatly upsets his parents—their family is very close-knit. I’m worried this rift will deepen and lead to more problems in the family. How should I approach these issues with my SiLs?

TLDR: I refused to be an usherette for my SiL after her inconsiderate treatment toward me. Now both SiL’s are shunning me and my husband wants to ban them from attending his med school graduation. Husband’s family is very close-knit and I’m worried these issues with his sisters will cause a rift in the family.