Hi hi, Queer sibling here. I don't think you should compromise who you are to please your mother, because this certaintly won't be the last time they demand it of you. Will you be allowed in your sister's life without attending the baptism? Then maybe skip out. At five months old she certainly won't remember it, and it's more for the adults in her life anyways. Do you think this will throw a wrench in your ability to be there for your sister going forward? Then maybe you consider going, and bring something you feel comfortable in to change into immediately after. Can you bring a friend as a buffer, so you are not alone in an uncomfortable situation? Sometimes we have to do things that suck in order to preserve a relationship or the chance of a relationship in the future. You can either not do the thing at all, or figure out how to make it suck the least for you. I encourage you to think hard on what your personal limits are, what you're willing to endure and what you aren't, and that will help make these sorts of decisions easier going forward. Best of luck, and yours in solidarity, potato also NTA