This is true
There’s so much to unpack
The OP needs to not compare - baby’s needs may not be the same as dSS needs, but that’s doesn’t take away her one needs , or baby
It is common for many families to divide and conquer when they have multiple children / jobs / responsibility… not everything goes down the line same same - but everyone needs to feel equally valid and respected …
DP and dSS need support to broaden their horizons so that they can feel close to others, as a family amd not just rely on each other
And this will allow the OP to feel supported - baby too
Op talks a lot about jealousy … but the relationship between parent and baby doesn’t happen overnight
Yes - DP needs to support her now , but not everyone bonds on day1 … and many fathers find it easier once their child is past tollderhood
Relationships evolve and many mums sacrifice years to their babies , only for them to grown up and find their absent fathers more interesting, balanced
It’s not all over yet . The baby is only 4 months old
The dad can still develop a lovely relationship
And DSS is soon to be a horrible teen
Things will change with the little ones
But mum can ask for help and that will be interesting revelation time