They won't be lonely! Just my opinion...
Many people ask about adopting (Or purchasing) littermates. We have a standard answer that we do not recommend littermates be brought to the same home. Keeping them in contact is amazing if you are lucky enough to have that option. The biggest issue with littermates is their socialization can be stunted since they have had a playmate their whole life and they don't need to look for outside entertainment - I mean isn't that he reason we had the second child? To keep the first one busy?
Littermates seem like such a good idea and with cats it works. Cats are independent and don't need another for comfort or as a playmate, but dogs actually will rely on the sibling. We see this often with housemates, but even more with littermates, they come to daycare and don't know how to play with other dogs, they rely on their sibling and if their sibling is otherwise engaged with another dog, they get frustrated or even a bit territorial. If two dogs from the same house come to daycare, we may allow them to play seperately with the other dogs so they get a chance to play without the distraction of their sibling. And with some siblings, it is a requirement for them!! They have to have time on the floor alone becuase the sibling doesn't like to see them playing -sometimes assuming that they are being too rough or are being roughed up. It's actually very interesting to watch the interactions. It's not always a male/female or an older/younger situation either.
As they grow older, we see that the interactions that happen with littermates in a daycare situation only gets more complicated. Commonly referred to as a "two headed" dog, they run the same circles and act exactly the same. Which can be overwhelming for other dogs they are playing with who doesn't love the "two for one" policy the littermates have.
While adopting two at the same time seems to make sense, consider getting one now and one at another time (we waited 2 years - just like our kids). If you have the idea that two is better than one because one will keep the other one busy: that is NOT how it works. You will have two now that need your time and attention. You and your dog need time to bond and establish a relationship. If you do get two, you need to schedule seperate play time, walks, and training time. You can google Littermate syndrome and get a litany of information both for and against. You can talk to people who have litter mates and said they never had an issue. Yes, while all that is possible it is typically the exception to the rule.
For the record, we do offer help with recommedations for type, size and age of dog that would be a good fit (in theory) for your currrent dog we know. If you have asked Kyle, he is an amazing resource when it comes to who your dog likes.
Or he will tell you get a boy cat. Because almost everyone loves a boy cat!
P.S. Many rescues have rules against adopting out littermates. Those are the ones we choose to work with and recommend. It's a good way to know that a rescue is looking for what is best for the dogs they adopt out and hope to not get returned.
P.P.S - This doesn't apply to older "bonded" dogs. These are dogs who have been raised together and would be devastated to have to live with out their sibling.