i get what your trying to say, but it doesn't really make sense if you think about it. yes i know that Ar is used to refer to your dad's younger sibling. But that doesn't explain why banarasee and banarot refer to boramed as Ar. Actually they really shouldn't call him Ar to begin with. Okay, hear me out on this one....regardless of whether boramed is related through the grandfather or the grandmother, he, as well as boramee, is related to banarasee and banarot through their mom not their dad. thus technically, they should call him na not ar. the only person who can call boramed and boramee Ar is junjao, since they are related through her dad. Get it? I hope that was clear enough.
anyways, as i stated before, if the grandfather and boramed's dad's are brothers, then boramed and boramee would have full rights to use the last name, surasit, since it would be their dad's last names, cause brothers typically have the same last name, do they not? (unless of course the brothers had different dads - but that's just more speculation.) nevertheless, we know that boramed and boramee do not use surasit as their last name, as evident since episode one, when they were arrested. banarot stated that boramee's last name isn't surasit. and technically, banarot and banarasee's last name should really be jaisad (or something like that) since that's their dad's last name, but they use their mom's last name of surasit instead. thus the only one who has the right to use the name surasit is actually junjao - and i guess that's why she's the number one heir...
therefore, it makes more sense that boramed and boramee are related through the grandmother...
banarasee banarot surasit