It is a difficult decision but Sammy is still a pup & you still have the joy of adolescence to go through so personally I wouldn't want to commit myself to another dog yet awhile!
I do like terriers; our house-guest, Rooney, is a Cairn & a sweeter, more easy-going little chap you couldn't wish to meet
but he is about 8/9 years old & has been well trained & socialised so he fits in with our dogs very well. I have also considered a Border Terrier as our next dog; we are both undecided as I know Borders have a reputation for taking themselves off & disappearing down holes after rabbits (a bit like Spike but fortunately he is too large to actually get down the rabbit hole - not for want of trying!!
) Apparently they do have good temperaments though.
Personally I would
never kennel a dog outside; I really can't see the point of having dogs & shutting them away from the family activities; JIMO & I know our dogs are horribly spoilt!!
Good luck with whatever you decide - remember should your sister forget again to shut the baby-gate you'll have
two dogs creating mayhem in your house!!