SpoiltSpat and the changing table

Y’all.

I literally cannot.

I allowed a visit to our house, if you can call it that, so that BF’s parents could celebrate Xmas with the kids. Despite Spoilt constantly giving her two cents, and me being undermined by ALL the family members at the table in a discussion about my knowledge of my son’s grades, everything was going sort of awkwardly ok, till SpoiltSpat...inexplicably...followed BF into our laundry room. I set up baby’s changing table and all her toiletries there, it’s also where all my dirty laundry is, and she proceeded to trail BF oh-so-intimately in there whilst holding on to LO’s little foot (she’s just turned three months).

I don’t know if I can explain the next part.

I feel personally violated.

She proceeded to like, please forgive me to describing it this way, suggestively lean up against the side of the changing table (my dirty fucking underwear at her feet!!!), while squeezed herself next to BF.

BF meanwhile has baby buck naked. You know, changing her diaper?

At this point I’ve run into laundry room, gawking at her, and she gives me the side-eye continuously till BF starts addressing me, at which points she grabs LO’s foot, squeezing it asking BF if it doesn’t have his brother’s genetic defect.

I want to throw up.

I want to break up with SO.

That was my happy place, and she SHIT on it. I change baby in the sunlight every morning, and giggle and laugh with her...

She’s done this shit since the beginning, hypothetically pissing all over any claims to BF, no matter how Jocasta-like.

BF claimed to want to cut contact after seeing how distraught I am...and then backpedaled majorly.

For reference, Spoilt, in the past, in any chronological order, has:

Bent my then-newborn’s pinky backward (two days old, kids),

blocked my head out of the family photo,

badmouthed me to BF,

stared at my (then) very slender waist in totally-raging jealous CBF that could shatter Aquaman’s pectorals, like instantly,

acted like she can’t understand a word I say (and)

actively criticized my parenting to my face, in front of others, ad nauseam.

What do?

Thoughts?

EDIT- Thank you all so much for the support, and great advice. I am subscribed to JustNoSO and and starting my research!

I also want to edit and mention I kind of lost it, I not only sent her a scorch-the-earth text (no curse words, but just listing all the stuff I found reprehensible), and because I am not always cool, I tried to destroy the changing table in a controlled environment with an ax. This is the last straw for me, there have been years of backhanded comments, oblique insinuations, and overt Jocasta-bombing of BF in my face.

Thanks y’all, even if I don’t deserve your help, just talking about it feels better...