Just out of curiosity, does she ever contribute gas money or any of the other expenses that seem to fall to you and you alone?
You pick her up. Apparently sometimes you agree to outings that include her children and taking them places you would not likely go yourself. She never "remembers" to bring her own suit. She tries to 'guilt' you into not enjoying the pool on those occasions when she has no suit available because she "forgot" hers.
Just exactly what does she bring to the table? It sounds like this is a totally lopsided 'friendship'. Everything is for her and her convenience and you are just the transportation and supplier of necessities.
Given that she also has a history of never returning anything you've loaned her, I don't understand why you need this particular, specific, uneven friendship.