I agree with the first posters that you should speak with a priest about this.
Second, I would ignore any advice from the poster who insists that it is Catholic teaching that you should go to the wedding while simultaneously adamant that you should not ask a priest about it. That’s obviously shady.
Third, if after you’ve consulted a priest, you decide not to go, you own your mom an apology because you initially told her that you would go and that sort of thing is upsetting. You need to talk to her and make it clear that while you don’t approve of, and cannot celebrate, the nature of her relationship with this other woman, you still love her very much. Then once you have said that, walk the walk, and make sure that you remember and honor her, and show her your love.
Lastly, I think if you decide tell her that you are not going, there are basically two likely outcomes. Possibly you and your mother will “agree to disagree” on the nature of homosexual unions and you will continue you have a good relationship while respecting one another’s beliefs and boundaries regarding such beliefs. This is obviously the preferred outcome. The other unfortunate possibility is that one or both of you will decide that your ability to have a relationship hinges on one’s complete acceptance of the other’s world view. It sounds from your post that you do not feel this way and that you wish to have a relationship with your mom, even if she continues to have homosexual relationships. However, if your mom does not feel the same way, I promise you that attending her wedding “to keep the peace” will not ultimately appease her. In that mindset, unless you offer 100% approval of what she is doing, then you do not really support or love her. Unless you renounce your Catholic beliefs of marriage and agree that there is no sin in homosexual relationships, you do not really support or love her. I’m NOT saying that this IS her mindset. You will have to talk to her and figure that out. All I’m saying is that IF that is her mindset, you are really wasting your time and compromising your soul for no reason by attend the wedding. It isn’t going to end there. You will be expected to “support” her indefinitely.