Does your mom have access to your
MDE? If not, she won't really know how many people are on the reservation....have your son distract her when you check in (if son is too young to let in on the secret, suggest mom and son stand aside while you go to check in) so they don't say "party of 5" or something....and when they seat you at a 6top table, just shrug your shoulders if she asks why you have such a large table, lol. Or....time it so that your sister walks up in line just behind you and surprises mom in line! Texting would be a big help in that timing....sis could stand inside the building next to TLT and just come up behind you as you walk into line. I seriously doubt there is a way to assure that two reservations would be sitting together......I know they place them in the order received, but if by the time you finish reservation #1 (online or on phone) there could have been a half dozen others finishing their reservation before you're done with #2.
What great kids mom has to celebrate her big birthday with her at WDW! I just turned 50 earlier this month and I hope someone does the planning for a big trip for my 70th!