没有必要提高声音，一个人有一个人的说话方式，你喊是喊不过她的，该怎么说还怎么说，该制止她就制止她。然而

“Let me fininsh". 是你在请她“让你说完”，这句话把自己放在了从属地位，难怪她不听你的。

可以试试，第一次：let me finish.  第二次: I'm talking.  第三次：Please don't interrupt. 

这三种说法语气越来越重，如果她还插话，听她说，然后看着她的眼晴说：Jane, we are here to talk to John because we couldn't decide what's the best way to deal with ...  I need both John and you to know what's my suggestion and it's very difficult for me to talk when you constantly interupt me. Could you do me a favor not to interupt me anymore?  I'll let you know when I finish.

说的时候脸上永远带着笑, 把声音压低了，目的要明确,，态度要平和，立场要坚定。如果她是无心，大概一次之后就行了。如果她开始狡辩，就重复说 “Could you do me a favor not to interupt me anymore?” 直到她同意。