A new name on the list of names we have been considering is the name [name_f]Susannah[/name_f]. We love how it is uncommon but familiar, has cute childhood nicknames [name_f]Susie[/name_f] and [name_f]Anna[/name_f]/[name_f]Annie[/name_f], and honors my mom [name_f]Anne[/name_f]. There is only one drawbacks for us.
DP has an aunt named [name_f]Susan[/name_f]. She is a very sweet and kind woman, but we wouldn’t be naming our baby after her. We will probably see her around 5-8 times a year at family gatherings and I’m wondering if it would be confusing. Also, she has children one of whom is also pregnant. Should we leave her name and its variations to her closer family members?
No, [name_f]Susannah[/name_f] and [name_f]Susan[/name_f] feel distinct enough to me. The syllable stress being in different places definitely helps. I think [name_f]Susannah[/name_f] is gorgeous and it sounds like it’s a great fit for you!
[name_f]Susan[/name_f] and [name_f]Susannah[/name_f] feel distinctive in terms of sound and feel and I don’t think it would be weird, but there is definitely the potential she and other family members may think it’s an honour name
I wouldn’t keep from naming her [name_f]Susannah[/name_f] just bc of Aunt [name_f]Susan[/name_f]. There are plenty of su- related names to go around! I love the name - [name_f]Susannah[/name_f] [name_f]Claire[/name_f] was on my list for the girl we never had! 
Personally I would double check with her pregnant child, just so you dont accidentally end up using the same name, or making them feel they can no longer use that name. But that’s really the only reason why I would be cautious about it.
I agree with above, might be worth giving your DP’s cousin a heads up (since you see her and her mother regularly) but I don’t think it needs to be more than that. Among my siblings and cousins four of their girl offspring have a [name_f]Bel[/name_f] or [name_f]Belle[/name_f] in their names. It’s not a big deal for anyone! I wouldn’t personally think a [name_f]Susannah[/name_f] was related to a [name_f]Susan[/name_f] anyway. [name_f]Susan[/name_f] goes with 50s names to me, and [name_f]Susannah[/name_f] with biblical but if it was brought up, you can always just be lighthearted about how it helped seal the deal in choosing it having a lovely [name_f]Susan[/name_f] in the family. I had [name_f]Susannah[/name_f] on my list and was going to use [name_f]Suki[/name_f] or Zannah as a nn.
I think it’s different enough that using it wouldn’t be weird… however, I do understand your reservation about saving it for her kids. I’d maybe mention it to the pregnant cousin…
My sister named her son a name that was a family name for our cousins (from the other side of their family). She didn’t realize one of my cousin’s hoped to use that name for sentimental reasons one day… My cousin was sad but she was never mad at my sister or anything like that. So, if you don’t want to check in with closer relatives to [name_f]Susan[/name_f], I think if it does happen to cause hurt feelings, they’ll mend quickly.