If we assume that they are not blood related,and they just like to hang out with Cera,i dont see any problem with Mr.threehorn.But if we assume that they were adopted,I think grandpa is fine.But if they were adopted than where they went?I prefer to think that they visited their grandfather,and then went back to their parents,and one of their parents is Topsy's older child.
I don't think that we should judge them by their colors.First of all,it would be pretty confusing if they were like miniature ceras running all around.With different color,they look more unique to me,and you can tell them abart,which is not a disadvantage.
Also,Tricia is pink becose Tria,but she is still Cera's half sister.The twins colors also looks like the color of Cera's father,so i don't see a problem with that.
Maybe the animators just wanted to make change with the other horns.If we not assume them blood related,it suggest that they are not even the same species becose of that horns,just another ceratopsian with three horns which grow its horns when it's younger,like Coahuilaceratops or whatever.I don't think Topsy would realy consider diferent species as a family.