I would see this as an opportunity to become a Catholic Christian witness to your daughter. I would focus on this part first. This can include as much detail as you seem appropriate for a 9 year old, but the theme should include that Catholic’s and protestants both love Jesus, what her aunt did was a sin, but you forgive her for sharing information that is not true.
As for your sister, I would not be too agressive with her. Her prediction of your being Catholic seperating the family will come true even though she initiated the battle. I would address the situation that she should never go behind your back, and that you had a long talk with your daughter.
If she cannot accept your religion that’s fine, but please keep any comments about it to herself and you will do the same. If she continues, you do not have to threaten to keep your daughter away, just limit contact without her knowing.
This is a time for you to show what being a Catholic means–taking the moral high ground when people offend, correcting firmly with the goal of peace. Rememeber that your daughter will be watching this, and absorbing. You also want your daughter to continue to share what your sister or anyone else says.
The OP’s daughter isn’t nine, is she? She a teenager, involved in Teen Life at her parish.
When your child gets to be a teen, she is going to start hearing messages that you didn’t screen ahead of time. I really think the thing is to be there for her to step in when, as in this case, the son or daughter is being subjected to messages that are unwelcome, aggressive, and purposefully covert. An independent teen may only want assurance that her parents are
not going to chide her for rudeness or disrespect if she tells her aunt to take her message and her secrecy and bug off. A teen feeling more vulnerable, as the OP’s has been feeling, may want more active help in setting boundaries.
That is what I would make this: that is, I’d take my teen’s lead concerning how to handle these unwanted religious advances, much as if she were being subjected to unwanted romantic advances. I would not want her to be mortified at how it was handled, but to feel as if she had my help in handling it her own way.
I am guessing, however, that the OP’s daughter is going to say, “Yes, tell Aunt Mary that both you and I want to make sure she never corners me like that again. Tell her I consider it a personal attack, that I’ll be strictly avoiding her if she ever does it again, and that my parents back me up on that.” In that case, what is really in order is an apology, since the sister should have been able to see that she had crossed the line from evangelizing to high-pressure coercion tactics where the target is cut off from allies that might help her resist unwanted persuasion.
The most important thing, however, is that the teen does not feel that confiding in her mother will probably lead to her mother taking over the decision about how to handle the problem under discussion. She ought to feel that confiding in her mother will be a source of wisdom, strength, and practical help in discerning how to handle her own problems.
She should not be given the feeling, even by accident, that she was the source of family strife or that she somehow “ruined everything” in a family that used to get along pretty well or that you, the parent, went totally beyond the way the young adult wanted to handle what is a matter between her and her aunt. She should not feel that her mother went ballistic and made the daughter’s problem into a big feud between mom and Auntie, instead.
The matter could be handled by the teen on her own, if she feels free to cut these things off from the start if this ever happen again. If the young woman wants to do that, then I’d let her do that, because she would know that she is beginning to gain a position of an adult confiding in another adult in the handling of her problems. That is the most likely to lead to her feeling she has a lifetime confidante in her mother, because her mother won’t take a request for advice or the venting of a bad scene as an invitation to come in and run everything. Still, the teen also ought to feel that her parents will run interference with the “elders” in the family if she wants or needs them to do that, since those elders might not take the authority of the younger generation seriously when it comes to setting boundaries.
If I had younger children, however, I’d certainly tell my sister that she’d crossed a line by targeting a child behind the backs of the child’s parents, and that I would not tolerate it. With a teen, however, I’d want to be careful not to do this in a way that the teen would take as disrespectful to the teen’s growing capacity to think for herself and take care of herself, lest I ruin her willingness to confide in me as she becomes a fellow adult.