Hi,
Yes you can upgrade directly from 5.2(1)N1(6) to 7.0(5)N1(1). ISSU is possible, provided you meet the requirements. You can copy the kickstart and system image to bootflash, then run the "show install all impact" command to see whether or not ISSU will be possible with your configuration.
Here is a document supporting the statements above: http://www.cisco.com/c/en/us/td/docs/switches/datacenter/nexus5500/sw/upgrade/705_N1_1/n5500_upgrade_downgrade_700.html#pgfId-640981