Calling someone my friend is beyond communication and helping each other to achieve a particular thing. That person should have my interest at his or her heart. It shouldn't be one way kind of friendship, like it's one person showing care and support, that means there's an error to what the friendship should be all about.
And as for your friend wanting you to come for a sister's wedding, it shouldn't be compulsory. You are his friend and not his sister. I don't know about you but I am good in dropping any friend that gives me unnecessary pressure
As much as friends are vital part of our lives, it shouldn't turn to a burden.