Hi Sunny D,
She may not be "stewing", she may just have decided she'd had enough of you and whatever you did and she "moved on" by moving you out of her life.
Chosing not to have anything to do with you or your family at Christmas may have been the healthy choice for HER. Just because YOU thought it was nothing and YOU wouldn't have made her decision, doesn't mean she is stewing, can't forget or her choice was wrong.
I think it's MUCH harder to drop it when the other person thinks what was hurtful to us is silly or no big deal to them and invalidates my feelings. If my feelings are acknowledged, even if you wouldn't feel the same way in the same situation, it makes it easier for me to "forgive and forget". If I am hurt or offended and "you" tell me I am being silly, or melo-dramatic, or you don't think I should feel the way I do.... well then it may take much longer for me to "get over it".
I don't work on YOUR (global you, not sunny d you) timeline. I work on mine.
Yes, "things get said when heated" but that doesn't mean I have to take someone's hurtful words and their excuse of "in the heat of the moment" over and over again. Sometimes I've had enough. Maybe it wasn't ONE thing you said, but the constant onslaught of little tiny digs that my sister has thrown at me for 40 years (well, ok she couldn't talk until she was 2, so 38 years). So that one time when she said something small, it wasn't that ONE thing..... it was the relationship and her as a person.... that I was done with. Ooops - look at that.... projected my sister right into your story. Enough about me.
Did you apologize for what you said, even though you felt it was no big deal?
I notice it's March and you're still "stewing" over the fact that she didn't get your kids anything for Christmas 3 months ago. Is that too long? Not long enough? Without being you or her it's hard to tell.
I try to forgive. I try to move on quickly. I try to not do anything that will hurt someone and if I do I apologize, even if I don't really understand what I did. I try to have conversation so that I can understand and not replicate my behavior that may have hurt someone, or I figure out why I did what I did and if I should keep this person in my life.
And then I make ammends or I dis-engage.
Sometimes that is a matter of moments or hours. Sometimes it is months or years to work through hurt.
Depends on the person, the offense and the partnership to heal.
Sometimes you have to wallow to wade through, because if you run and splash in your hurry to get over it, the muck gets in your face or all over your clothes. Sometimes you wallow, so that you can find the clearest path out. Sometimes out of the wallowing comes greater healing than ignoring or burying.
I hope you work things out with your family member. Since you know she was offended by something you said, you might want to be the bigger person and give her a call. Tell her you just found out you upset her and you're sorry you upset her. Tell her that wasn't your intention and you'd love to mend fences. Don't mention Christmas. Just mention that she's family and you'd like to fix it.
Just my $0.02
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ETA after reading your SWH: You put her in her place. She put you in yours. if she's your SIL, then your BROTHER (???) should have gotten your family gifts. The responsibility doesn't fall completely on her, right?