I think that what your sister is saying this is a sign they do get the feeling but won't admit or don't understand it fully.
When I was in school and had not came out yet I got people talking on me. some start bulling me saying.
"so who is the new boy" or "should we call you martin"
I did not mind this, as this was how I wanted to be seen even if I was not taking seriously.
Now thinking back I belive alot of the people who have said these things have deep down seen it comming.
it wasnt really that huge of a surprice when I changed again. some where surprized but I also reach reactions from some of them who wasnt surprised of this at all. even for those who had seen the signs early there had not been much to do, I wish there had been more suport but thinking back my life was diffrent. there was not knowlegde and suport about these things as being trans like today and I did not have as much change to go seach for the suport myself as I do now.
today if someone is threading me badly I can find words, aguments and suport which I did not had back then where I did not even thought I had a word which was good enough to be taken seriously.
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I hope this helps alittle.
p.s. I dont think 16 is too young to understand
I been talking with all kind of age group of being trans kids from 8 and adults, I also do education for the moment on schools and I diffrently belive a 16 year old can get the thing of being trans.
The issue is often the parrent and the sociaty around, Even if you talk to a child who is 8 who get it, it may come home to its parrent and been told it wrong and being scolled. for me this is the biggest challenge. kids and teens are in my opinion more easy than adults but also more limited in there own freedom.