yeah when DD was younger, mostly siblings came along. You invited them knowing that 'little Hyley has a sister Lucy', and that the Mum would bring her too ...
Its either invite siblings, or expect that the child wont be able to come.
Once they are older, its much easier to invite just the friend, as most parents drop and run anyway ... or arrange for another parent to take their kid
That is what happens with us anyway. So for example, it is my DD's birthday party tomorrow, and their are 2 that siblings are invited (because she is good friends with the younger sister also) but various others where the siblings are not.
If in doubt - call the parent. Most of htem will appreciate you just asking, offering to pay for younger child, or advising that you cant go.
Hope you have a good time ... little kids birthday parties are fun (well, unless you are the parent arranging ... then they are a nightmare )