Quite honestly, I could be your sister in this scenario (minus the potentially toxic significant other!). If you’d indulge me for a minute I’d like to float a theory… not an excuse for what’s clearly caused you sorrow, but more of a theory on the very harmless potential cause of this sudden drifting away.
You mentioned your own little ones are 5 and 3. I remember when ours were that age; it’s exhausting but you’re also often/usually/generally at the point of both being verbal, both being able to hold your hand and walk, neither being in full-fledged school to the point of homework, and both at a stage where you can pop on a show and take a lovely hot shower.
Presumably, from what you said their ages are, your sister WAS in a position similar to yours with her own children. The one year old… that’s the thing I’d bank on being this sudden-distance-causing factor. Going from two children to three is often a tipping point for moms everywhere, and not in a bad way necessarily! I believe you stated her oldest is 9 now. While you’d sort of figure that older = easier, the truth is that those “middle grade” years can also come with a boatload of other needs, largely related to Mom I Need To Talk To You. That’s when homework hits… or friends need to be discussed… or it’s not just your “Mom Friends” kids’ parties you need to attend, it’s your actual child’s friends who are having social lives. They have bigger feelings and a vocabulary to discuss them. They want books from the library and an art lesson that ties up your phone and they dirty clothes like you thought only toddlers could. And you begin realizing how swiftly the years are flying by.
Our children are 10, 8, and 7 months right now. That’s one child reading classic literature and wanting to chat about it and begging for a set of watercolors. That’s another child who wobbles between working on stunning handwriting and just wanting to play with cars on the floor. You’d think the 7mo baby would be what keeps us flat out (and in some ways it is), but I’ve often pondered how the little years are more about keeping bodies alive and the older years become more about keeping brains and hearts alive.
I suspect your sister feels that you have it “together” and she doesn’t. I suspect she’s probably sleeping less than you are and has been for a while. I suspect perhaps that trip to the post office was all her mind and energy could handle, and that maybe dropping by a lovely-sounding lunch for your child’s birthday was just going to be too much for her to handle in one afternoon. I have a feeling that someone who was formerly Poised and Gracefully Hospitable and Had Life Sorted is probably feeling like having to facilitate conversation between you and her partner’s family would just be like landing a rocket on Mars (even if it wouldn’t be.) It might be something as small as I Don’t Have Plates for Everyone for Snacks. (Obviously an easily-solved problem, but summer + baby + two other children = non logical mother, frequently!)
In a nutshell? Your sister might not even get dressed many days. That post office run might’ve been in sweaty spit-up-covered yesterday’s outfit. She might be soaking in the tiny milestones of childhood. Her emotions are probably still swinging rapidly. Little things seem like big things and big things (such as a birthday!) seem unimportant when compared to a toddler’s teeth or something like that. She might feel like her child’s birthday isn’t as magical as your child’s would be, she might feel like that’s the new baby’s fault, and she very well might feel guilty. And if she’s watching any online posts she’s probably under pressure to “savor every moment” or “only eighteen summers” and so on.
I say it with love and no assumption. My family/friends probably wonder where I’ve gone and if I care. I do care. I do care greatly. But the third backyard barbecue this summer pales in comparison to a snuggly Saturday in pajamas actually getting some SLEEP and not having to Be On and Talk. I love talking. I’m just too tired. And I don’t want to miss a moment of this… our last baby, those last moments of firsts. I love my family and I love my friends and I don’t know what time it is a lot of days and I stress over how our older children are handling all the huge life changes and I’m hoping my family will have grace for when I emerge from a cloud of bottles and diapers and snuggles and sweaty contact naps and delicious little snuggles and have - Lord willing - yet another toddler to send off to the sandbox while I indulge in adult conversation.
Three - with older ones OR younger ones - is a challenge. You sound organized, thoughtful, and beautifully engaging. I wonder if your sister is not organized right now, thinking mostly about the life she’s building in her own little family, and a bit absorbed in the days which are so very long while the years are short. It also sounds like she has a bit of a network that doesn’t include you… and while that hurts you, I promise it can be beneficial. Multiple circles = better failsafes and more support that isn’t all hinging on one connection.
I bet if you give her grace she will emerge triumphantly ready for coffee or a lunch… in a year or more.