I cant get over "hating" my SIL and my hubby says it's redic because I know how she is and shell never change?

I've bee with my husband since high school; we've been married less then a year but have been together for almost 6.5 years. I obviously know his family very well ect. Yet his SIL still shocks me with her stupidity. When she was 3-4 she was diagnosed with a "comprehension" problem. Basically it means that it takes her longer to learn/understand things that other people would get on a first/second try. This is something that had my MIL given the proper attention to she could have easily over come. Maybe she would have always not been totally on the ball 100% of the time; but she certainly wouldnt have had her mom make excuses for her her whole life. My MIL is the type of person that nothing could ever be wrong or different about her kids. So she never addressed the problem however anytime my SIL does anything redic (which is all the time0 she uses her "disability" as an excuse. My SIL has a 3 1/2 year old and is about to have a second. She is just so frustrating to even think about. SHe makes up redic lies for NO reason about "nothing". I can't take it anymore. I know it is how she is and how she was raised but I dont understand how a 26 year old woman with almost 2 kids can not have to be held accountable for the things she says/does. I just cant help it. And it's not like this has no affect on my life. She has been borrowing my hubby's car for 2 weeks now becuase hers was mysteriously crashed into in her front yard. I end up watching her monster child becuase she's "tired" and my MIL just shows up with him and I cant really be like get out and take him; when I am a SAHM. What can i do. And NO i am not moving.