I have like 3 questions hope that's all right :]
1 What is Will's relationship with the twins? 2 how did Stanford get sucked into the portal? 3 what happened after he got sucked in?
1. Short answer: they have warm and close relationships (not really family, but more as friends).
Will care about them {but their first meeting wasn't that good} and they are in kinda good relationships with him. For example:
Will is in love with all this theatre stuff and he was desperate not being able to visit it for 30 years, while was stuck in Gleeful's mansion walls. BUT! When Ford returned from the portal, the demon discovered that he actually can come out of the mansion, but only with Ford (consequences of the broken deal blah-blah...) But William won't step over his ego just to trying to ask Ford if they can go to the theatre.
AND. Here it comes. Twins know all of that (Will loves theatre, but won't ask Ford), so they act like THEY want to go to the theatre with Wil, that's how they convinced Ford (he "fell" for it, yeah).
ALSO. Interesting fact - they LOVE when Will is making them some new clothes or spending time with them, so they're pretty close (and the demon protects them from Ford).
2. Short answer: they've argued pretty badly with Will, so the demon was someone, who trapped Gleeful beyond the portal for the next 30 years.
Long answer: gonna be in the next post {but first - we're gonna have a good sleep}.
3. Short answer: he got transformed by the power he has gained.
Btw, there's not yet gonna be any long answer, cause we're still developing this part of the story.
P.S. Hope this post answered {almost} all of your questions!