Should I ask my parents to throw down for my sister's part of a bachelorette party?

Months ago, my friend got a group of girls, including me and my sister, to agree to $240 each for a bachelorette weekend in late Sept, so she booked it.

My sister is in rehab for a (relatively small) alcohol dependency. Although she will be out by the bachelorette party, she has decided not to go to things like this until she can navigate her way around social situations- totally understandable. (Unfortunately, her whole life- so seperate from the alcohol problem- my sister has an incredibly frustrating and horrible habit of excitedly committing to something then almost always bailing even when someone else has already paid for her to go, so I keep having to seperate that from this situation.)

The money for the house is due September 11th. My sister has no money- my parents support her completely.

My friend's fiance has said he'd pay her way, but I dont think thats right, theyre not exactly swimming in it especially with the wedding a few weeks after. She also doesn't want to ask her friends to pay the extra $40pp because most of them are tight on money as well.

What should I do:

1. Tell my friend to ask my sister to still throw down?

2. Tell my friend to at least ask the rest of the party to pay the extra?

3. Ask my parents to throw down for my sister's part?

4. Let her fiance pay?

I'll say right now what I won't do is ask my sister to figure it out- its complicated.

Additional info:

-My husband and I have lived overseas for the past 1.5 years and although we did plan on coming at this particular time of the year for the wedding, we are staying for a whole month to spend some good time with family. A reason im iffy about asking my parents is that even though we're staying for a whole 4 weeks, no matter what I say I get the vibe that they think its completely centered around the wedding and bachelorette party and staying in the city with my friends (Ex. with a genuinely non passive aggressive "I hope we'll have time to visit with grandma while youre here.")

-Although the friend and my sister worked together for a while, she's more my friend than my sister's. Although my sister and I hung out only every few weeks, she's afraid of feeling alone, so my friend wanted her to feel included and have it be something I could share with my sister as well.

-I didn't ask her to invite my sister or ever imply that I wanted her to come though I was happy she did. (Honestly I maybe should have told her to ask for the money up front with her because I know how unreliable she is.)