In general, interrupt routines should be short enough that nesting is not necessary unless you have very, very strict timing requirements (which serial communications doesn't).
the timing requirements for serial communications when receiving are "very strict". If you do not get a byte read before the next one reaches the stop bit you are lost.

To Ragu
you need to redesign your software. If you need to fool around with interrupt enabling inside ISRs you will need a megabottle of aspirin before you start debugging, and, when you believe it works there is no guarantee whatsoever that some timing 'coincidence' that will only show up at the end user will not blow the thing out of the water.

Erik