Hello Mijita and a warm welcome to the boards

When an aging and/or ailing parent also has BPD, that's exceptionally difficult. Often the expectations about how
you should be acting can feel painful and like people don't really understand the reality of the situation. We do get it here; these are such difficult situations.
Is there an in-person or zoom support group for people with parents with BPD? I am seeing a therapist weekly but feel the need for additional support.
Yes; check out the National Educational Alliance for Borderline Personality Disorder (NEABPD)'s
"Family Connections" programs. They have one for BPD, and one for suicidality. The waitlist may be longer for the BPD groups (I've been waitlisted for ~5 months already) than the suicidality groups. My understanding is they meet via Zoom and will be more skills-based (there is homework) than venting or co-rumination based.
The National Alliance on Mental Illness (NAMI) may offer in person groups in your area; take a look at their
support groups page to see the options.
Both NEABPD and NAMI are reputable groups, so I hope one or both of those gets you started in a helpful direction.
...
my mother has BPD. I am her only child and she is currently in treatment for stage 4 cancer. She has alienated everyone in her life (friends and family) and is completely alone except for me.
It sounds like your father is not in the picture -- is that correct?