Hello and welcome.
If you’re not intending to breed them (and even if they were brother and sister - full, blood related, littermates brother and sister), once the female comes into heat, the male will do anything and everything possible to try to mate her - and when she is in her receptive stage, she’ll be just as determined as he is.When we got her a brother about 1.5 years ago, she was physically active - she liked to do various running and jumping games in the back yard with us. We socialized her, primarily, by going to the local park every single day, rain or shine - and I still do. She's very sociable except somewhat aggressive toward, maybe, 1 in 10 dogs that she encounters. We call the new dog her "brother" because we have no intentions of letting the two breed and we wanted them to develop fully (physically) so we didn't have them spayed/neutered. They live indoors on our back porch that's open air to our living room but cordoned off with a gate/fence so we can keep them out of the LR when we have guests and when our cats are roaming the house (they really get excited around those cats), geez. Anyway, making a short story long, we love them dearly.