Is it normal that my (33f) partner’s (41m) sister’s therapist (UK based) is his sister’s “best friend” as well as her therapist and she’s in very regular contact (texting and calling) him and his mother?

I’m beginning to feel uncomfortable with my partner’s sister’s therapist texting and calling my partner and his mother so much. Shes always calling him and “checking up” on his sister. I told him I think this is unethical and it makes me feel uncomfortable, but he told me she separates her friendship with her work and since she’s “married” and has a disabled child I shouldn’t feel worried… I don’t actually know if she’s married or if the child is real.