Let me try and explain why I had suggested to change the boarding point.
As you might know, 2 lower berths are reserved for every coach offering sleeping accommodation for male above 60 years, female above 45 years and pregnant women on production of relevant medical certificate, provided they are traveling alone. (Except First Class and First Class AC; FC and 1st AC)
Now on consumption of all the available berths as earmarked at ARP, the status moves to Not Available and any further booking is done against the concerned sectorial quota.
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in this example, the booking is been done against PQWL while for the second ticket you have booked the booking will be done against GNWL. Now assuming someone who has booked a confirmed berth against #lowerberthquota cancels his/her ticket. This ticket will be kept withheld and will not be immediately released into the system. Instead bookings will keep on happening and this vacant berth would only be released during the preparation of chart to clear any deserving passenger on first come first serve basis.
Now, I assume that I as a senior citizen have been successful in getting a berth against #lowerberthquota from ATP to MTJ (which originally comes under PQWL). Now due to certain obligation, I am unable to make the journey and I cancel this ticket.
There are 2 more person eligible for #lowerberthquota who has booked ticket after me. However, since I was the last person to get the berth after which the quota got exhausted the availability status had moved into respective sectorial quota. That is for a person who wanted to book a SBC-NDLS ticket got it under GNWL while the person who wanted to avail a berth against #lowerberthquota from ATP to MTJ got it under PQWL with respective position 2 and 1.
Now, during the preparation of chart, who will get my berth? The answer is number 2. The reason is that just like PQ and HO, this particular quota also applies from start-end. The system will first search for is any eligible candidate is present who has done his booking from start-end, if not, then may be from a different boarding point but which comes under GNWL, if still unavailable then it will start picking up passengers on serial order irrespective of quota and boarding point.
In this case, even SSPN will do but SBC would have had given you even better chance.
Kindly note, in an event where no confirmed berth against #lowerberthquota gets cancelled, this will move as per normal cancellations. Even then, this looks to be a sure shot confirm thing. However, the berth then which might be allocated remains doubtful. You would need to negotiate onboard or to the TTE for change in berth.
Citing the age of your mother, you can even apply to SBC/SWR to allocate a lower berth against HO.