Wow!! I..just...wow!!! They got some brass ones, now don't they??!! I would not respond to them in any way shape or form as it will cause them to keep sending these over the top messages. I would tell your mom that they did send them, that you don't appreciate it because it is too soon for this kind of interaction, and to tell them to please not send you any more texts. I am going to assume that your siblings are living with your dad..have they received these messages too? The fact that this man is telling you your dad didn't make her happy, only he does is beyond rude..how dare he disparage your father. And the steppies comment? How old is this girl? They are both so in your face it is ghasting my flabber in ways that have never been done!
Poddity pod to this.
The mother might have been unhappy in that marriage but for the children thats usually not really obvious HOW unhappy. To them their "Happy Family" split up and they have to get used to these new circumstances. (I am not sure if thats the same in OPs case though)
And the new Boyfriend writing a SMS like that, is definetly NOT a way to speed that process up.
He might have had good intentions and wanted to introduce himself, but simultaniously implying that OPs dad was "no good" and OPs family made her mother unhappy is really not the best.
As others have suggested, talk to your mother, OP. Depending on how you get along with her, you can open up about how you feel about her new beau contacting you and the suble implication that was in his text.
A colleague of mine had a similar experience a few years ago as the OP.
Her Dad decided he wasn't happy in the marriage anymore and left his wife. Shortly after he was with someone else.
My Colleague referes to her dad only as "That man" today for the way he handled everything and spoke ill about his ex-wife.
Not everyone is able to move on after only a few months.