Brother & SIL pregnant 2mo after I announced my pregnancy... WTF??

So, back story:

My half brother is 25 (7yrs younger than me, no college degree, no real stable job, completely financially inept, renting a questionable house; i.e. in ZERO position to have a baby) and knocked up his gf, making the first grandchild of the family. They got married about 6mo later. Not only was it an "oops", but they also almost completely excluded my Mom from just about everything to do with the pregnancy/birth, which hurt her tremendously (all while still asking her for financial support). The whole experience seems to have left a bitter taste in everyone's mouths as far as grandchildren are concerned.

So, fast forward a little under a year, and my husband and I (already married, already both graduated college, financially stable, own our own house, etc) get pregnant with our first, as planned. The reaction from the family seems pretty muted/jaded and it was a big disappointment to me. However, they seem to have warmed up as the pregnancy progresses.

During the course of my pregnancy, my brother and his wife have shown signs of jealousy towards me (don't know if it's because I'm getting the attention now or what). To my shock, I found out yesterday that SIL is pregnant AGAIN, due 2mo after me. The first thing she said was, "Well, we didn't do it because his sister got pregnant, we just wanted a small age gap"... They can't even afford the kid they have now!

What's worse is now my Mom is freaking out because she just doesn't have the money to help support another child (because she's the type who would starve before letting her family go without), and my brother knows this. Whereas his father (Mom's exhusband) has done NOTHING to help out. So, instead of considering retirement after a lifetime of hard work, my Mom and stepdad are burdened with my brother's irresponsibility and bad life choices. I want to advise her to make him man up to his own choices and cut them off, but I know she'd never do that.

I'm such a mix of bad emotions right now - flabbergasted and infuriated with my idiot brother, concerned for my Mom, and a little bitter that they took something with their irresponsibility that was supposed to be mine (having the first grandchild), casting a negativity over the family, and they're doing it again!

Thanks for hearing (reading) out my rant. I'm just beside myself and just don't know what to do.

*EDIT*

My issue is not so much that he had the first grandchild, but rather how the circumstances of it jaded my family and I feel like my pregnancy is being tarnished/overshadowed by it, despite me doing everything "the right way". Plus, all signs pointing to their second pregnancy being a result of jealousy over mine (there's a lot more evidence for this than I elaborated on in the post). However, that's all *secondary* to my concern for my Mom's well-being.