I actually disagree with this. Yes, families can help each other out on the odd occasion but something as regular as monthly rent or upgrading her engagement ring just seems like a reason for Molly to hold it over her head if she ever needs anything in the future, like when Tommy goes away and she wants Zoe just to move in with her despite what's going on in her life. My parents have always had money but I've wanted to build a life for myself on a midwife's wage so I wasn't ever in the position of feeling like I'm indebted to anyone.
I also think if someone's paying your rent or helping with it, it takes that incentive away from your own work. Poor Danny has been working away and missed actually even seeing the new house, would that feel worth it if you knew your fiancés sister was topping up your rent payments anyway?