JensSis,
I've struggled with those same concerns and while I can't say I've found a group that doesn't endorse religion, I feel that it was because of the area and the limited exposure I've had to the meetings, not Al-Anon itself. I've been to many AA meetings (as a visitor) and 90% of them never addressed God, only a HP (higher power) which for me is the earth and her beauty and power. Its the love and freedom I feel standing in front of the roaring ocean, while the waves crash on shore. Its the comfort and love I feel when I am falling asleep in the arms of the man I love. Its the comfort I feel inside despite the thunderstorm. Its the butterflies I get when I see an adorable fluffy bird outside my window singing her perfect little song. Whatever "THAT" is, that is my HP and that is who I hand all my pain and sorrow to.
You're right. You should def. go to Al-Anon for you and not your SIL, but like the woman above me, maybe try out a few different meetings and see what you think. They say "take what you need and leave the rest".
I really admire the way you explained how you feel and what's going on. Very easy to understand and it seems like you have a lot of personal insight. Your concerns are very normal and more people will be along to share their experiences.
Oh, and nice to meet you.
I'm Holly.
<3<3