I’ve been a user for a few months now and a relative, who is also a user, says by now I shouldn’t have access to Sarah’s quests anymore because I have over 100,000,000 sP in my vault. I still have access to her quests, I actually just finished them, and I was told I should come and report this here in case there is a bug going on.

I believe it was changed at some point so that you can have access to Sarah's quests for the first 6 months after you join, even if you have more than the sP limit. I see that your account is only about 5 months old, so that's probably why.