Hi,
You just do like this in the sign in button click event:
Control uc = LoadControl("YourLoginWebUserControl.ascx");
YourPlaceHolder.Controls.Add(uc);
And then, remember to re-create the control in the following postback's otherwise if the usercontrol was raising any button click events or so, they will not get fired in the code behind.
That's it :-)
Regards
Joachim