Is it obligatory to clarify that the habit of collecting old siwak has no relation to religion, or would the questioner be sinful for not doing so, fearing ridicule?

What you feared by opening the door, you have opened, and you have harmed yourself with it. This is due to the devil's mockery and manipulation of you by introducing you to whispers and doubts, which you were previously advised to stay away from, ignore, and completely disregard. Whispering (waswasah) is a severe illness and a chronic disease, and indulging in it leads one to confusion, doubt, distress, and harm. Therefore, beware of whispering in general, and in this matter in particular. You should not pay attention to what the devil makes you imagine, that what happened with your brother might be a form of mockery of religion.