Quoting Kyleigh'sMommy12:" caitlin is just like my daughter. im going to get complete HELL for this but it is the only thing that ... [snip!] ... sister did it with my nephew (even though moms on here say not to) and he is perfectly independent and happy and 10 years old "
Well... keep in mind it's not her child, so she should consult with the parents before trying this. But if it was her own child, I would 100% agree with what you said.
OP, talk to the parents and see if they are open to trying CIO. I don't think it's something that should be done behind their back, but I feel like CIO works well if done properly. Also, you could look into baby carriers/slings, so when you are needing to do other things, she won't be so upset and you have your hands free.
DD1 was just like that, I couldn't imagine having her PLUS a twin that wasn't able to get the attention she should get. I did CIO the moment I was able to, and even before the recommended 6 months I would lay her down and let her cry for about 10 minutes at a time. (Judge if you want. I had PPD and no treatment for it, plus a colicky baby, I needed some peace once in a while)
I wish I'd had a baby carrier back then.