Engaged after a month????!!!

So my crazy sister sent me a text at midnite last nite to tell me that her and her boyfriend are engaged. OMG! She has only been dating this guy for less than a month AND when they started "hanging out" she said they weren't dating...she just wanted him as a friend with benefits. She hooked up with him the day after breaking up with another guy she dated for 3 months. She goes thru men like crazy. AND then yesterday morning she called me asking me to cosign on a loan of $10k so THEY can buy a trailer......but the trailer needed fixed up alot. SHe has no credit and he has horrible bc his house was foreclosed. NO WAY....the longest she has kept a job for was 4 months. I lied to her about my credit bc there is no way a trust her to make payments. What is she thinking??????