First of all I think a second SV is a great idea, I just added my third this summer. I can also give some firsthand experience to why it is best to follow all the good advice given here and quarantine them separately.
My 3 year old girls are half sisters and about 2 months apart in age. Abby is a F3 and Frosty is a F4. When I got them, Abby was 3-1/2 months old and Frosty was 5-1/2 months old. When I got them home I had put Abby in my bedroom and Frosty in the adjacent bathroom. Frosty was terrified and found a space between the cabinet and the wall to hide. I didn't even realize there was a way into this space from the toe kick area. After a day I decided it would be best to allow Abby in the bathroom to make Frosty feel safer. Of course this greatly slowed down my bonding with Frosty. Frosty did not let me pet her for 6 months, and even then it was a quick pet. Abby wasn't a problem and had quickly bonded with me. Now both are very bonded to each other and me, but it took a long time for Frosty to come around.
Now for the health issues. I took Abby to the vet a couple of days after coming home, Frosty didn't go because she was hiding. It turns out Abby had giardia, so they gave me medicine for her but they won't give me any for Frosty because they hadn't seen her. In the end they passed the parasite back and forth and I had to treat Abby 3 times and Frosty 2 times. If I had continued to quarantine them separately, I probably would have only had to treat each of them once and saved a lot of money.
I would definitely get his half sister, but definitely quarantine them separately.
Sent from my iPad using Tapatalk