You husband has children.
Children get ill. You chose to marry a man who had children.
What a ridiculous comment. The OP has the right to do what she can to avoid catching her stepkids' illnesses, especially as she's immunocompromised.
OP, I get it, both because I'm immunocompromised too, and because this used to happen at our house when DSD was small. DSD was frequently full of a cold, and I did feel her mum had a duty to say something beforehand, which she never did. DH would go to pick her up and find her with a streaming cold, by which time it was too late to reschedule the visit without disappointing DSD. (In our particular instance DSD's mum was so selfish I sometimes used to wonder if a part of her was glad to get DSD's germs out of her own house for the weekend, but that's by the by.)
I wouldn’t really bother to inform family we had something like a cold before visiting tbh because it’s just a cold and at this time of year, lots of people have colds. They don’t tend to bother the vast majority of people, it’s just something we all deal with.
And right now, what seems like a cold could actually be Covid. How irresponsible. Even pre-Covid, your attitude is selfish. Why should other people have to risk catching your family's colds? And what about those who aren't the 'vast majority', who really suffer in the aftermath of colds? Like the OP, I am immunocompromised and colds make me really unwell, weeks of bronchitis-type coughing amongst other things. Not everyone who catches a cold gets off with a bit of a sniffle, you know.
It is something you’re going to have to get used to though now you have a child of your own
How's the OP supposed to do that? She can't just decide not to be immunocompromised. Are you saying she should just resign herself to getting ill a lot so other people don't have to bother restraining themselves from sharing their germs and those of their kids? Again, even pre-Covid, that would have been a selfish attitude.
You sound like those people who boast about 'never having had a day off sick in their lives', blissfully oblivious of the umpteen people they've made ill in the process.