To each his own, Buscee. Some people, well, a TON, possibly MILLIONS worldwide, have loved the show for seven years.
And now to actually answer the question in an unbitchy manner...
EverythingisRENT - I think the answer lies in why your mother was devastated about your sister's wedding.
**SPOILER**
The daughter doesn't get married in the end. The mother does and the daughter just runs off with her fiancee to tour Europe together before they decide on marriage.
**END SPOILER**
So if any of that mirrors your sister's wedding scenario, then I would go see Hairspray or Spelling Bee, both of which are lots of fun. And ignore those who think you should not see the show simply because they didn't like it. I think Mamma Mia's reputation speaks for itself. Personally, I find it difficult to see the show and NOT have fun unless you go into wanting to hate it, but that's just my opinion.
If you think your mother is sensitive enough about weddings that anything involving a wedding may set her off, then I think you have your answer.
"What can you expect from a bunch of seitan worshippers?" - Reginald Tresilian