Hi Tracy,
How many cMs do you share with them ? Whether using my heritage or Ancestry , they both show cMs and %.
Enter the cMs that you share with your 18% match into a dna painter to give a more of a guestimate/ probabilities
To give you an example of the huge differences of using % instead of the cMs into DNA painter , my probabilities drastically differ if I use the % that I share with my known parent's half sibling ( who I share 17% with ) rather than our shared cMs
Putting in my shared 17% percentage in dna painter gives me a 32% of me being their half sibling or full Nephew Niece. And 68% of being a half Aunt uncle, half nephew Niece and so on ( and will give me a "prediction" that I may share 1265cm, I actually share 1162cMs )
"68% Great-Grandparent Great-Aunt / Uncle Half Aunt / Uncle 1C Half Niece / Nephew Great-Niece / Nephew Great-Grandchild
32% Grandparent Aunt / Uncle , Half Sibling Niece / Nephew Grandchild"
Then if I put in my shared cMs ( 1162cM) instead of my 17 % that I share ,changes the predictions/ probabilities percentages by a huge amount- it is then only 7% that I am a half sibling, full Nephew/Niece and a 93% that my match is my half uncle, full 1st cousin and so on, and my % will also change to 15.6 % instead of the 17% that both ancestry and myheritage has given us.
"93% Great-Grandparent Great-Aunt / Uncle Half Aunt / Uncle 1C Half Niece / Nephew Great-Niece / Nephew Great-Grandchild
7% Aunt / Uncle † Niece / Nephew † Grandparent, Half Sibling, Grandchild"
https://dnapainter.com/tools/sharedcmv4If it were me , I'd build a tree around them, and if at all possible persuade your close relatives to test , your siblings if any, and if known , second cousins on your father's side.
Very Kind regards .
Ps that's very high for a second cousin (your known cousin on your mum's side). You say only , "only 7%". I share with all my known full second cousins 4% (one is 5% but I share over 300 with her unlike my other second cousins who are all in their 200s)
Are you sure you don't mean that it is your first cousin once removed ? Ie your parent's first cousin, or your first cousins child. I share that ( 7%)with my first cousins once removed , as too my child shares that with his first cousins once removed too .
7% puts it very loosely around 521 cMs which basically there's only a 3% chance of being second cousins, totally possible of course, but the odds are really low.
Only asking are you sure , because I know of quite a few people in my family that incorrectly call their 1st cousins once removed their second cousins, and 7% seems very high for 2nd cousins.
Kind regards