If you're going to a relatives house and staying overnight and only need to wear the diapers at night for bed wetting then place the diapers inside of clothing in your luggage and then when you go to the bathroom to shower or change clothes take the luggage in the bathroom with you to put on the diaper.
Then you can place the wet diaper into a plastic bag and place it back into your luggage along with your dirty clothes and then dispose of the diaper on the way home or when you get home.
If you need to wear the diapers during the day you can also place the extra diapers inside your clothes in the luggage but to hide the diaper you're wearing wear a snap in the crotch T Shirt which can help to support the diaper and hide the diaper from view should your shirt ride up.
Wearing boxers or briefs over the diaper can also help to conceal the diaper and they would likely never notice you're wearing a diaper unless they pat your butt or are looking closely at your bottom which many people would not be doing.
Since your relatives do not know about your diapers then they will not be looking for a diaper bulge or anything to determine if you're wearing diapers.
Change in the bathroom and then throw the used diapers out after leaving the relatives house by placing them in a sealed bag inside your dirty laundry luggage or your plain luggage.