Cleidocranial dysplasia to bring the shoulders together.
Cleidocranial Dysplasia - A Case Report
Persons with the syndrome sometimes lack collarbones which gives them hypermobile shoulders. I understand that cats (except lions) also lack collarbones which accounts for some of their axial hypermobility.
Your person can flex and fold the pelvis because he or she has hypermobile pelvic joints.
There are 3: the pubic symphysis in front and the two sacroiliac joints. If these are hypermobile the sides of the pelvis can move relative to each other. There is not a muscle to pull the sides of the pelvis together like the person above had pulled her shoulder together, and so your character would need to fold them in manually then fold them back out.
That does not feel good when it happens to hypermobile people and some have surgery to lock the pelvis in place with metal. That is how your character is biomodded: it does not hurt her to flex in these ways and she can pop back into place.
Mostly she uses her ability to rescue children and animals who have fallen into tight spaces. And she plays bass (sometimes sitar!) and sings backup in her band.