It is actually for a friend, so less info. But she suspects that one of her family members know what she is watching on youtube and maybe also about Whatsapp messages. Do you think that is possible? The family member has is in a foreign country and has never accessed her phone. And she has not installed any apps that the family member has sent. What do you think?
Only if they have the password to her Google Account or her device is set in the Family Friendly mode (which I find very difficult to happen, otherwise she wouldn't be able to use WhatsApp), tell her to try changing her Google Account password, also, she can install Kaspersky Internet Security (https://play.google.com/store/apps/details?id=com.kms.free) to check if her device contains any Spyware or related software. I hope this helps her, if you have any doubts or want further information please let me know by commenting.