No, I like it/think they sound cute together also!
I honestly don't like same letter siblings so I feel they are too similar.
I don't think they are too close at all. I think they make a great sibset!
It looks like I'm the voice of dissent. Samuel and Sebastian just seem to matchy to me, and--trust me--I've thought a lot about this exact issue! I truly love both names on their own (I was seriously considering using both of them myself and wound up having a Samuel), but I do think they're a bit too much as brothers.
That said, I guess it partly depends on what you nn them. I was partial to Samuel nn Sam and Sebastian nn Sabe (rhymes with Gabe), and I really think those don't work well together. Sam and Bastian is better, I think. Still, I wouldn't (and didn't) do it!
They're very sweet together and not too matchy at all :)