Kids not wanting to go to their dads…

I have a 11 year old girl and a 13 year old boy from a previous marriage.. they go to their dads every other weekend. They use to have no problem with going but about maybe 6 months ago he bought a house and lives their with his girlfriend her daughter who is 14 and there new baby who is 1. The past couple times my daughter has called me crying begging to come home because his girlfriend’s daughter is sooo mean to her! She’s talked to her dad about it and he does nothing! Well just recently both of my kids cry to me about going because they say the girl and now their dad is extremely mean to them! They say everything they do he yells at them.. even if the Girlfriends daughter does the same thing and he does nothing to her! They both tell me that he is extremely nice and sweet to her but just does nothing but yell at them! So now not only does the daughter make them feel unwelcome.. he does too! Apparently he is even mean to the little baby at times.. but never ever the 14 year old! He lets her get away with anything!! I’ve told them I’m going to talk to him but they both beg me not to because they say it’ll just make him even more mad and yell! I don’t know what to do! I do not want to send them anymore.. I just don’t know how to go about it without him screaming more at MY babies 😔