My personal opinion for what it's worth, is that you need to get this sorted now and get a behaviourist in to see the behaviour and work with you - as just on a forum we can't see the dogs reactions and signals etc Also I think that getting a behaviourist in will help you read Holly's signals as you say she is very quiet and when you heard her snap you were surprised, but she would have been given off signals before this
from what you have said I would say it sounds like guarding, but not just you, but also Benson as she chased Bella away from curling up with him - I would also agree that there is a good possibility that Bella hasn't been socialized properly
For what it's worth I would not have introduced them for the first time in a house but in the very least a garden as this was your sister's house and not yours, my advice for now would be DO NOT let another dog come into your own home / garden until you have got Holly and you some help and that includes your sisters laid back lab Bella
What was Holly's reation when you fussed Bella and turned your back on her after the event?
When you first got her did you socialise her as at 9 months?
Good luck and please get some proper help before this escalates, as now she has bitten another dog you can't be 100% that she wouldn't do the same to another or a human. Sorry some people may think I am scarmongering you, but I don't mean to, just want you to see the worst scenario if you don't get help now