My DD2 started severely stammering at 2y6mths literally overnight. Had about 3 sessions with a private SLT while waiting for the NHS appointment to come through. Followed advice which was around slowing things down for her. Lasted 3-4 months and then totally disappeared and has never returned. She is now 4y8mths.
This has coincided with DD2 beginning to make her presence felt. She is now mobile and 'taking' DD1's stuff. DD1 is very clingy with both DH and I as well as saying 'I cross with DD2' frequently. She dislikes me holding DD2.
My Ds's stammer emerged at the same age to your daughters. I was worried and kept an eye on it. After he was 3.6 months it was getting noticably worse and he was getting frustrated about not being able to communicate easily. We got our GP to refer us to our SaLT service, but it took 5 months from referral to treatment. We then had weekly NHS hour sessions for 18 months doing the Liddcome method which is v v effective with toddlers. He hasn't stammered for nearly three years, sometimes get a little muddied, but don't we all. It was really intense but so worthwhile. He wouldn't have got better 'naturally' and from my understanding it is only a small minority of children who outgrow it without SaLT intervention. Take your concerns seriously and seek professional help if you are worried. It was the best thing I have enabled for my son, the ability to talk freely, fluently and confidently - it's doubled edged sword these days as he never stops!