If I am added as 2nd holder, will shares be considered my income?

If my mother adds me as a 2nd holder in her shares, will the value of these shares become taxable income for me?

Addl info:
This addition will not happen on or around my marriage.
I am already a nominee in these shares -- will this make any difference?

Assuming you do not sell it before a holding period of one year,I do not think there is any tax impact.
Unfortunately different companies have different procedures but as a nominee at least you do not have to go through a long winded procedure for transmission of shares. Still by adding your name as a second holder,you need to have the necessary application on a stamp paper and notarised or authenticated by a bank manager.
Clarify these aspects with the registrar and transfer agent (RTA) of this particular company.
But I am warning you, if you add yourself as a second holder you may have to open a separate account in both your names to demat these shares. Else in the present form you can demat these in the demat account in the name of your mother.
Dividend receipts are tax free in your hands since it has suffered dividend distribution tax in the hands of the company.