When assigning a patient to a twin node in pedigree the twin status gets lost, and pedigree editor may crash

Steps to reproduce:
1. create a new patient A with a simple family (mom, dad, child (proband,patient))
2. save
3. create patient B and add them to the family of the patient A
4. in the pedigree editor click on the patient A (proband) and create a twin
5. assign patient B to the twin node
6. save
7. load
8. observe that nodes representing A and B are now regular siblings, not twins

Alternatively:
6*. click on the mother's node and try to create a sibling (Update: or perform just about any other operation which changes pedigree structure)
7*. observe console error msg "Uncaught Incorrect positioning: Separationbetween nodes is to small"
8*. clicking on the mother's node no longer responds
9*. save and close the pedigree editor and reopen it
10*. observe that the mother's parent nodes were created (they we not there prior to saving and closing)

Another scenario:

• repeat steps 1-5, 6*-8*

• click on Undo button

• pedigree disappears; patient A and patient B are unassigned

This anomaly is present only if the proband's sibling node is a twin.
The problem was not found in PhenoTips version 1.3.10