My boyfriend kisses his sisters on the mouth ew?

Anonymous
My boyfriend has two little sister that are like 12 or 13, well I've noticed lately that whenever we leave to go somewhere or something, they kiss. [he and his sisters] on the mouth. It bothers me because ill be the one kissing him after that and I don't want other peoples germs. its gross. I don't mind his germs obviously. its just weird to me, they are too old to be kissing him on the mouth. anything else think this is weird? how should I tell him that it bothers me because I know he'll get upset if I just tell him. is there a gesture I could do or say that I don't want his sisters germs? lol thanks
Updates
+1 y
okay, its not about the germs really, let me restateee this, its the fact that his kissing his sisters on the lips that bothers me.
My boyfriend kisses his sisters on the mouth ew?
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