Friends or Not

I am best friends with Tiffany. Tiffany has a sister named Trish. I have known Trish for about 5 years. Our friendship bloomed but never flourished. She has a daughter, Keli and 4yo grandchild and dramatic (bipolar) step daughter , Ariel with a sickly infant. Whenever you in invite Trish she generally will ask if her daughter can come. I feel like if I invite one daughter then I should invite Both. Also, an invite for 2 becomes 8. I feel like I am out in an awkward position, if I wanted to invite her daughters then I would have , why ask??? It has become easier just not to include Trish but I feel like I should. The last time I invited her over, she asked if her daughter (and family) could come, I said “not this time”. She accepted the invite and cancelled at the time of the get together. I don’t understand why this is so difficult! Yes I know, I have a ton of thoughts about Trish and her family.
My question is how to stop this indecisión about our friendship. If it were not for Tiffany then I probably would not even give this another thought.