Sharing bed with ma sis

I almost sleep beside my sister, she is 4 years younger than me, i got no bad feelings before, that my friends told me that its not normal,told me that she is not younger anymore, you can't sleep with her, she is ' vayasu ariyichathanu ' like stuff,

im living in a middle class household, were there are only limited rooms, cause we had to share each other, does anybody have experience like this, or feels bad about this, share your thoughts fin.