I would suggest to limit your lo socialising as much as possible my lg had it over her first birthday she was generally fine with it but I caught it off her and I wouldn’t wish it on anyone I had blisters on my hands feet and in my mouth.
When my little one had this I was told to keep her social interactions limited until the sores started to heal over
I think it depends how poorly your child is feeling? My son has had it 3 times and each time, his symptoms have differed and varied in degree. I would certainly keep him out of direct contact of your sister if you can, just to minimise any chance of her getting it. I’ve caught it twice and have been fine but my brother caught it from my son and he was really poorly and his face and mouth were covered in blisters bless him!