Bonkers diversion! I was the sole breadwinner and went back to work full time very early on - but honestly, I can't say that experience suddenly made me less upset by familial problems!
I guess that poster was trying to say, it's not a big deal and if you were busier you'd take your mind off it. Except, it IS a big deal.
So I disagree with that on both counts :)
Maybe for very kind reasons, but the end result is that they have dropped out of their daughters life in favour of someone who is a relative stranger. There shouldn't be a feeling of replacement, but there is as that's how they're behaving at the moment.
Can you have a very careful and gentle conversation about it with your mum? With rehearsed phrases so you control the conversation and refuse to let it end up in an argument or them saying you're jealous and mean! Explain that you think they're wonderful for welcoming skneone into their family wtc etc etc, and you know they don't mean to but you miss them, especially your mum. Don't get drawn into discussing your nieces mum or their relationship or it turns into a competing and jealousy conversation which you won't win! Just keep it very nice and say you used to love doing X and you don't seem to get the chance these days, and youd love to do x, and for your ds to get to know her grandma, and how much you love her and want your Dd to have that strong relationship with her too.
Don't get your parents defensive as then they'll argue back or justify what's happening, your aim shouldn't be to push this girl out (as it will just alienate you anyway!), your aim is to get your mum back in your life... Can you remind her of any conversations you used to have when you both planned doing things together when the baby came? Not in a 'you didn't do it' way but a 'remember how much we wanted to do x, let's do it!' Way.
Oh and if that all fails, try the gentle making feelings known thing... Ie ask yourself over to do x, she refuses and you say sadly, 'oh ok, it's just I miss you', and then repeat invite, each time not getting stroppy but showing sadness, and perhaps even explaining that you feel sad because you haven't done anythjng with her for x months, and you miss her' etc.
It's about making your sadness visible, but not getting your parents into defend, justify or blame territory. Basically, if they feel defensive, or have to justify their past actions, people tend to strengthen their current behaviour and the more that happens, the more they get locked into continuing so they dont look wrong/ admit wrong. That's the way family rifts happen.