Is it strange for one to refer to certain Gods as one's father, uncle, grandfather, etc?
No, I personally don’t find it strange.
In my experience, I’ve seen people doing this for one of two reasons:
Simply to formulate a “heathen” name, or
Because they feel that their relationship with a particular Deity resembles a familial/romantic/friendly relationship
The first of these is exceedingly common in American Heathenry, and I have likewise seen it done by heathens from other countries. Devotees want to have a name that mimics the patronymic/matronymic naming patterns of Old Norse or modern Icelandic, and moreover conveys their feelings of devotion to a particular deity. While not Reconstructionist in any sense (these names are meant to convey one’s actual familial relationships), it is still a very popular practice. If you are interested in doing this, though, make sure you do the genitive form correctly at the very least! I wrote a post about it here.
Secondly, a smaller number of people like to use this terminology to describe the kind of relationship they have with a particular Deity, whether it be familial, friendly, romantic, etc. In my personal experience and the experience of many people I know, Gods can relate to us in ways that resemble human interaction.
In the corpus of Old Norse literature, we have a few examples of connections between Deities and humans described in personal and/or “human-like” terms:
Wife: (of Freyr) Ögmundar þáttr dytts
Husband/Lover: (of Freyja) Hyndluljóð st. 6
Friend: (of Þórr) Eyrbyygja saga ch. 3, (of Freyr) Sigurðarkviða in skamma st. 24,* etc.
"So dear to": (Freyr) Gísla saga Súrssonar ch. 18
"Fully trusted" (Fulltrúi): (with Þórr) Eiríks saga rauða ch. 8, (with Þorgerðr hörgabrúðr) Þáttr Þorleifs jarlaskalds, etc.
On top of that, many royal lines attempted to trace their blood back to particular Gods, though (thankfully) I haven’t seen modern practitioners attempting to assert that very often.
In short, historically speaking familial relationships were sometimes used, but usually the person using them was trying to assert a literal blood connection to the Deity. More common are labels like friend of, spouse of, dear to, or, above all, priest of. That being said, in this matter I am willing to put historical sources on the backburner for a bit and listen to people who say that their relationship with a particular God resembles that of a father/brother/mother/grandfather/etc. If a God can act like a friend or a lover to a human, I see no reason why they cannot act like a father or mother.^
You will find heathens screaming and moaning about “Christian influence” the moment anyone suggests a personal connection to a Deity. I don’t buy this. For one, we have the attestations I mentioned above. Moreover, my personal experience tells me that Gods reaching out to humans in a personal way has nothing to do with Christian expectations. I stopped believing in Yahweh as soon as I was old enough to understand (I was 8 or 9) and never expected or experienced a personal connection with him or any other God. The things I felt for Óðinn, then Skaði, then especially Loki almost twenty years later were unexpected, unlooked for, and sincere.
If we believe the Gods are real and that making offerings to them is possible and beneficial, it follows that those Gods have the ability to interact with and build connections with humans. Sometimes the best way to describe those bonds is by using human relationship terms or describing them in comparison with other interactions we know from ordinary life.
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*Though Sigurðr is a legendary hero, which makes close relationships between human and God much more likely, in his case
^ In fact, this seems even more probable given the hierarchical positioning of a God to a worshiper that most people accept, at least to some degree.
