Siblings of GAG, do you think it's acceptable for a brother to put sun tan lotion on his sister's backside at the beach or pool?

If you are at the beach or by the pool with your sister who is wearing a bikini (or nude, if you are at a nude beach), is it inappropriate to apply to sunscreen to her backside to prevent sunburn?

Or is this sort of skin to skin contact okay because of the context?

Siblings of GAG, do you think it's acceptable for a brother to put sun tan lotion on his sister's backside at the beach or pool?
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