Frustrated Step Mom

When my son (now almost a year) was a newborn my SS came down with strep throat. Being a first time mom I was highly concerned. I would not allow my SS to hold the baby or get very close to him at all. I received a lot of negativity about it from my SO's ex and even my MIL saying that "it's common and kids get it often" all because my SS said that "it hurt his feelings he couldn't play with his brother". I understand that, however I felt I needed to protect my newborn (3 weeks old at the time) and that it should have been explained to my SS why it wasn't okay for him to interact with his brother at the time, rather than me being judged. The hardest part was that my SS's mother waited until he had a sore throat and cough for over a week to take him to his pediatrician (which I had suggested be done the day he came to our home with a cough and raspy voice...and secluded my son and myself in the bedroom). I was made out to be a mean step mom, the only one that had my back was my SO and he got the grief from his ex worse than I did. 

Recently both boys came down with infantigo. We are unsure which one had it first, but as my son is now on the move and loves his big brother they are constantly in contact with each other. I noticed it on my son and took him to the doctor within the same week, he was diagnosed and treated with meds. The day after I noticed it on my son, my SS pointed out a spot to my SO, but his mother would not take him to the doctor as his regular pediatrician was closed for the holidays and she refused to have him be seen at a clinic. (She's a SAH and my SO works out of town so she is the one that takes care of doctor/dental visits). Unfortunately my SS rash spread without quick treatment, and his mom now has him running from my son like my little boy has the plague and is adamant that my son gave it to my SS (we really can't be sure which boy contracted it first)...however she had no problem letting my SS go to a birthday party (which I may have sarcastically commented about 😬). So now my son's feelings are hurt wondering why his brother won't play with him (even though they are both on meds now) and I'm catching crap for not making a big deal out of this situation even though, like strep throat "it's common and kids get it often". I'm a new mom and really don't want to get into an argument over who is in the right/wrong, but the way she has my SS behaving when he is at our home has me really pissed off. My SO has tried discussing it with her, but it hasn't helped and as hard as I have tried I am beginning to act negatively towards my SS for his behavior even though I know it is because of his mother.