His Ex and His Family

I really hate this but should I care, honestly?

My hubby's sister is "bffs" with his ex. That in itself is already an issue with me (because they used to hate each other before me and hubbs got together)

Not only does my hubby's ex refer to his sister as "sis" or "sis n law", but she also says that she is the aunty (of sisters kids) and refers to my husband's grandmother as "her granny" and is always posting old pics of them on social media. I'm weary that this girl will always be in the picture. How awkward is it to attend family holidays and parties with this girl present also!!! She had previously tried to break us up not once but twice and almost succeeded. I know my husband cannot control his sister and her life but we constantly argue over it. Should I care? ? Should I look past it? Advice...please!!!