My friend's older sister currently has temporary legal guardianship of her. Her father (lives in CA) said he would sign emancipation papers for her to be "on her own". Can this happen without the older sister having a say? Also, my friend's mom committed suicide last year and so she receives a check (I believe from social security), if she is emancipated will that affect her receiving that check every month?
You've really not provided enough facts to give a good answer. A lot will depend upon which state issued the guardianship (CA or NV?) though I suspect they will be pretty similar. In Nevada, the guardian has first say in what happens with the child, and Dad would have to terminate the guardianship before he makes those decisions unilaterally. And early emancipation requires a legal proceeding, so merely signing a piece of paper will not divest the guardian of her ward. As for the Social Security Benefits, it will depend upon the type of benefits she receives (and the basis for them) so she should contact the Social Security Office directly for that answer.