Licking of bed linens

Why does my border collie lick the sheets and comforters of my husband and son? She does not do this to my side of the bed, or my older son's bed. She stuffs as much of the material in her mouth as she can, until she is "caught", leaving a large wet spot right where the covers would come up to their chins. YUCK!!! She does this to clean sheets as well as soon to be changed sheets. She seems to know that I am unhappy with her, even with a gentle tap to get her attention. (She has been losing her hearing for a long time.) The dog is 7 years old, and otherwise well behaved. I have tried leaving her out of the bedroom when I am sleeping, but she just scratches to get in until I can't take it anymore and let her in. I have tried putting chairs or a suitcase on the spot, but this is inconvenient when my husband comes to bed. She did this as a puppy to my younger son, but now he has learned to keep his door shut. She just started this on my husband's side of the bed recently. She gets away with it because I go to bed several hours before my husband, and she does it while I am sleeping. Please help!!

Why does your dog have free run of the house at night?
   
No dog should be unsupervised in a home.
   
Crate her for her safety and the safety of your house. This will resolve this issue for you in addition to ensuring that your dog does not choke to death while you sleep.
   
Since she is older and has obviously never been crated, she will probably resist at first but keep at it and eventually she will 'get over it' and sleep in her crate.
   
Since she knows she is doing wrong and doing it anyway she is not trustworthy so honestly who knows what other mischief she could get into when she is unsupervised for such an extended period.
   
Do not think of crating as cruel.
   
Dogs are not rational decision makers. They have chewed wires and electrocuted themselves, started fires and choked to death on toilet paper and suffocated themselves with plastic bags stolen from the trash so you are protecting her from herself.