petey2428 you might be frontrunner for the most complicate problem situation of the year
let me try to go ahead step by step
If you do a domestic flight first and then want to stay in transit in BKK, you have to do international check in at your first destination in Krabi. Usually you can only check in internationally if you can check through your luggage to the international destination. But as your mom's luggage will be checked under the name of your sister (who is flying with you), she can check in internationally, will get a C.I.Q. sticker, go through immigraton in Krabi (note that domestic and international departure areas are usually separated). Upon Arrival in Bangkok, she has to follow the groundstaff holding C.I.Q. sign and at beginning of Termina B on the left side is the entrance to transit security, which goes directly into the international departure area
Now to your case: If you intend to accompany your mom to the international area in BKK, you would also have to check in internationally and do the same as above. However as you have technically left Thailand, you would have to go again through Immigration in BKK and enter Thailand (going from international departure to international arrival area is not a problem, just take the Elevator down). Not sure about your nationality and whether this creates an issue if you leave and enter Thailand at the same day. It is certainly not usual to do so and if you mention your Intention to check in personnel in Krabi, they will probably not let you do this
as for the Priority Pass Lounge: those are now called Miracle Lounge (as owned by Miracle Group; in the past those were called the Louis Tavern Lounge). You do not need to have a Priority Pass. You can as well simply purchase entrance. I think they are around 1000 Baht per 2 hours (the normal Business Lounges, the First Class might be more expensive). Look up on the Website where they are exactly located and which one has shower
Did I cover all your issues or did I miss anything?