Please I'm in a bit of a dilemma . We have a family ceremony coming up this December and i need to invite my sister's husband's Sister (Brother in law's sister)

my questions are as follows,
Who is my sister's husband sister to me

How do i prove my relationship with her to UKVI ? . We don't have any pictures together aside whatsapp conversations. call logs . I will also be part sponsoring her trip .
is it necessary to include the granted visa of my actual sister and her family to support the application ? .
i would really appreciate your help because we met all the requirements for a visit visa other but I'm struggling to find the perfect title to give her since i was told she can't be addressed as my sister in law and also how i need to prove she is my sister's husband sister

Your input is really appreciated .